What is KVM? Everything You Need to Know About Kernel-Based Virtual Machines

Virtualization has become a cornerstone in today’s dynamic technological landscape. It changed the way we utilize our computing resources. Virtualization means creating a software-based representation of something physical, web hosting servers, storage, networks, or operating systems. It allows servers to achieve greater flexibility, efficiency, and cost-effectiveness in managing and deploying IT infrastructure. 

But what is KVM? Among several virtualization technologies available, KVM (Kernel-based Virtual Machine) stands out as a powerful and widely adopted solution. Thus, KVM VPS is mainly used for web hosting servers leveraged with KVM virtualization technology. Linux is the operating system that supports virtualization technology. 

What is KVM?

In simpler terms, KVM is a powerful technology piece that allows you to run multiple isolated computer systems. It is known as a virtual machine (VM) that enables you to run various isolated computer systems on a single machine. 

Think of it like having several separate computers operating independently, each with its own operating system, applications, and resources, all existing within the confines of one physical machine. At its core, KVM is not a standalone application but a virtualization model directly integrated into the Linux Kernel. 

Glossary: A kernel is the core of an operating system (OS), acting as the central interface between hardware and software. 

Besides, KVM directly leverages the kernel’s inherent capabilities like memory management, process scheduling, and hardware interaction. This extensive integration means that when you utilize KVM, you use the Linux kernel and its functionality as a hypervisor.

Glossary: A Hypervisor is the software layer that manages virtual machines. 

In doing so, KVM takes it one step further, turns the Linux kernel into the hypervisor, and allows direct management of the host hardware resources (the physical computer) where the virtual machine resides, allowing for the most efficient allocation to the guest VMs. This intimate connection to the operating system improves KVM’s performance and stability, making it a powerful virtualization option for various use cases.

What is KVM Virtualization?

Simply put, KVM (Kernel-based Virtual Machine) is a technology to create and manage virtual machines. This virtualization capability of the Linux Kernel enables a single physical server or desktop to host multiple isolated virtual environments. Thus, virtual machines operate as separate, independent computers with their own operating system, applications, and a dedicated portion of the Host’s resources like CPU, memory, and storage. 

For your better understanding, let’s compare KVM virtualization with conventional methods like Type 1 Hypervisiors: VMware ESXi or Microsoft Hyper-V Server.

• Techtionary:
  • Type 1 hypervisors run directly on a computer’s hardware, offering high performance and security. Examples: VMware ESXi and Microsoft Hyper-V.
  • Type 2 hypervisors run on top of an existing operating system. Examples: Oracle VM VirtualBox and Microsoft Virtual PC. 

Other type 1 hypervisors like VMware ESXi and Microsoft Hyper-V are also bare-metal hypervisors, offering excellent performance. However, KVM’s tight integration with Linux provides unique advantages, particularly in Linux-centric environments. It benefits from the robust security features, extensive hardware support, and active development community of the Linux ecosystem. Furthermore, KVM is open-source, which can be a significant advantage in cost and customization compared to proprietary solutions.

You get the substantial benefits of KVM in the cloud and server environment. Performance and scalability make it well-suited for handling demanding workloads and large-scale deployments. The live migration capability, a key feature of KVM, allows for moving virtual machines between physical hosts with minimal to no downtime, which is crucial for maintaining high availability in critical server environments. The open-source nature of KVM kernel virtual machine technology also fosters innovation and allows for greater customization and integration within existing Linux-based infrastructures.

How Does KVM Work?

Let’s discuss the functional aspects of KVM. 

– Integration with the Linux Kernel

Fundamentally, KVM’s strength is its strong integration with the Linux Kernel. Unlike other virtualization solutions operating as a separate application on top of an existing operating system, KVM is a native virtualization module that plays a kernel role. This integration allows KVM to transform the standard Linux kernel into a type 1 hypervisor that runs directly on the system’s hardware. 

Imagine your computer’s hardware as a set of building blocks. The Linux kernel typically manages how your applications use these blocks. When you load KVM, it’s like giving the kernel a special set of tools and blueprints that allow it to take these building blocks and construct smaller, independent virtual computers (the virtual machines) that can run their operating systems. User-space programs act as the architects, using KVM’s tools to design and manage these virtual computers.

So, your initial explanation is spot on. KVM bridges user-space management software and the underlying hardware, enabling efficient and powerful virtualization within the Linux environment.

– Hardware Virtualization Support (Intel VT-x, AMD-V)

The backbone of this virtualization support is its dependence on its hardware virtualization extension offered by modern processors. Intel’s technology is considered the best one in the processor industry. It is known as Intel Virtualization Technology for IA-32, Intel 64 and Intel Architecture (Intel VT-x), while AMD’s equivalent is AMD Virtualization (AMD-V).   

These extensions introduce new processor instructions and features specifically designed to facilitate virtualization. Software-based virtualization would be significantly slower and more resource-intensive without these hardware extensions.

Here’s how KVM leverages these extensions:

• Enabling Direct Hardware Access: A new virtual machine starts under KVM. The hypervisor utilizes Intel VT-x or AMD-V, allowing guest operating systems to access and execute code on the physical CPU. It eliminates the hypervisor’s need to constantly translate and surpass instructions to a near-native performance for the guest. 
• Memory Management: Hardware virtualization assists KVM in memory management allocated to each virtual machine. Features like hardware-assisted page tables allow efficient and secure memory isolation between the Host and guests, as well as between different guest VMs. 
• Privilege Levels: The processor’s hardware virtualization features help KVM enforce different privilege levels for the Host and guest operating systems. This ensures that a problem or security vulnerability within a guest VM is less likely to impact the host system or other virtual machines.

KVM Components

KVM components fuel the power of kernel-based virtual machines. So, it is essential to understand them before knowing “what is a kernel-based virtual machine.” While KVM is the virtualization infrastructure within the Linux Kernel, it collaborates closely with other user-space tools, providing a complete and manageable virtualization solution. Here are its primary components:

– QEMU (Quick Emulator)

QEMU is a versatile workhorse in the KVM ecosystem. It acts as a user-space emulator, providing a relevant environment for virtual machine operations. 

Glossary: An emulator is a software tool that makes one computer behave like another.

In the case of KVM, QEMU is the virtual machine’s process residing within the host operating system. While the guest operating system communicates with the virtual hardware (virtual CPUs, memory, network interfaces, storage controllers, etc.) that QEMU emulates, KVM allows QEMU to spin off and delegate any instructions that can be executed in hardware virtualization (Intel VT-x or AMD-V) to the KVM kernel module, which helps KVM and the guest to run at near-native speeds. 

• QEMU: QEMU would play when more complicated emulation is needed and/or when various hardware devices are simulated. KVM is an attractive option because it combines kernel-level execution (near-native performance) and user-space emulation, providing great flexibility in running multiple guest operating systems.
• Libvirt: Managing multiple virtual machines through command-line interfaces becomes challenging. Libvirt is a virtualization management toolkit with a robust API, and a convenient management tool to provide unique ways to interact with virtualization technology. With its consistent commands, you can create, configure, start, stop, pause, and manage virtual machines. 
• Also, tools like virsh (a command-line utility) and graphical managers like virt-manager are built on top of libvirt, providing user-friendly interfaces for managing your “KVM kernel virtual machine” instances. Libvirt also supports remote management, allowing you to control virtual machines on different hosts.
• Virtual Machine Monitor (VMM): Regarding KVM, “Virtual Machine Monitor” or VMM refers to the KVM module in the Linux kernel. This is what we have breathlessly described as the hypervisor and is the fundamental layer working with the hardware, allocating resources to the guest VMs, and providing isolation between guest VMs and the Host.

As you explore VMM with respect to KVM, remember that the kernel-integrated code within KVM is responsible for executing the primary virtualization tasks. It is the layer that QEMU runs individual virtual machines on top of and is controlled through libvirt. When you understand these pieces: QEMU to emulate the guests, libvirt to manage the guests, and the KVM module as the VMM, you have a better picture of “what is a kernel-based virtual machine” and how it serves as a reliable and efficient virtualization solution.

Benefits of Using KVM Virtualization

KVM offers a compelling set of advantages, making it a popular individual choice. Also, many cloud providers and businesses prefer KVM because of these benefits. 

– Open-Source and Cost-Effective

KVM’s open-source nature is the most beneficial aspect of the Linux kernel, it excludes licensing fees. Moreover, it reduces the total ownership cost compared to proprietary virtualization solutions. You have a robust and enterprise-ready virtualization platform without any software acquisition costs. In addition, a large and active open-source community around KVM contributes to the ongoing development, security updates, and unlimited online resources and support. The open source community enables innovation and sustainability.

– Performance and Scalability

KVM runs directly within the Linux Kernel and leverages hardware virtualization extensions (Intel-V and AMD-V). With this, guest virtual machines can execute code with minimal overhead. This makes KVM ideal for running demanding applications and workloads that require significant processing power and low latency.

Scalability is another strength of KVM. It can run many virtual machines on a host (single Host), but it can also scale across many hosts in larger virtualization environments. Its scalability creates a strong foundation for cloud infrastructure and enterprise server consolidation.

– Strong Security with SELinux and AppArmor

In any computing environment, security is foundational, and KVM can utilize the robust security features built into the Linux kernel. Additionally, KVM can take advantage of security-enhancing capabilities like SELinux (Security-Enhanced Linux) and AppArmor.

These Linux Security Modules (LSMs) offer mandatory access control structures, allowing the specification of strong security policies, which would enable virtual machines to be separated from each other and the underlying Host. The ability to control what resources are accessible and what inter-process communication (IPC) is allowed for each process and application can further solidify your security model in KVM, making it a very secure virtualization platform. KVM can be a trusted platform for sensitive workloads and multi-tenant use cases.

In any computing environment, security is foundational, and KVM can utilize the robust security features built into the Linux kernel. Additionally, KVM can take advantage of security-enhancing capabilities like SELinux (Security-Enhanced Linux) and AppArmor. These Linux Security Modules (LSMs) offer mandatory access control structures, allowing the specification of strong security policies, which would enable virtual machines to be separated from each other and the underlying Host. The ability to control what resources are accessible and what inter-process communication (IPC) is allowed for each process and application can further solidify your security model in KVM, making it a very secure virtualization platform. KVM can be a trusted platform for sensitive workloads and multi-tenant use cases.

Setting Up KVM Virtualization on Linux

A few steps are given below before you embark on using KVM virtualization on Linux OS. First, let’s discuss what your system demands!

System Requirements

Before diving into creating and running virtual machines with KVM, verifying that your hardware and software meet the minimum requirements is crucial. Here’s a breakdown of the essential system requirements:

• Operating System: You’ll need the best Linux distros supporting KVM. Popular ones are Ubuntu, Fedora, CentOS, Debian, and Arch Linux, which have excellent KVM support. Ensure your kernel version is relatively recent (typically 2.6.20 or later, though newer kernels are recommended for the latest features and improvements).
• Processor with Hardware Virtualization Support: As we know, KVM heavily relies on the hardware virtualization extension offered by the CPU. You will need an Intel processor with Intel VT-x technology enabled in the BIOS/UEFI or an AMD processor with AMD-V technology enabled. Check if your processors support these features through the operating system’s command line. For example, on Linux, you can often check the /proc/cpuinfo file for the vmx flag (Intel) or svm flag (AMD).
• Sufficient RAM (Memory): RAM gives the space for your machines to run. So, ensure that you set up a high amount of RAM. Ensure your hosting infrastructure has enough RAM to simultaneously accommodate the host operating system and all guest VMs. MilesWeb offers high size RAM and bandwidth with Linux VPS hosting servers for your scalability needs. 
• Sufficient Disk Space: You need adequate disk space to store the virtual machine images and related files. The required space will vary depending on the size and number of virtual machines you intend to create. Consider using external drives or dedicated storage for your VMs if you plan on hosting many or large virtual machines.
• BIOS/UEFI Configuration: You must ensure that hardware virtualization (Intel VT-x or AMD-V) is enabled in your system’s BIOS or UEFI settings. This is often turned off by default, so you might need to access your system’s firmware settings during startup to enable it. Consult your motherboard’s manual for specific instructions on accessing and modifying these settings.

Steps to Install KVM on Linux

Getting KVM up and running on your Linux system involves installing the necessary software packages and properly configuring your system. The specific commands will vary slightly depending on your Linux distribution. Here’s a breakdown of some popular distributions:   

On Debian/Ubuntu: Open your terminal and use the apt package manager:   

sudo apt update

sudo apt install qemu-kvm libvirt-daemon-system libvirt-clients bridge-utils virt-manager

qemu-kvm: This package provides the core KVM hypervisor.

libvirt-daemon-system: This is the system-level service for managing virtual machines.

libvirt-clients: This provides command-line tools like virsh to interact with the libvirt daemon.

bridge-utils: These tools are necessary to set up bridged networking for virtual machines.

virt-manager: This installs a user-friendly graphical interface for managing KVM virtual machines.

On CentOS/RHEL: Use the yum or dnf package manager:

sudo yum install qemu-kvm libvirt libvirt-python libguestfs-tools virt-manager bridge-utils

or with dnf:

sudo dnf install qemu-kvm libvirt libvirt-python libguestfs-tools virt-manager bridge-utils

The package names are similar, with libvirt encompassing both the daemon and client libraries.

libguestfs-tools provides tools for accessing and modifying virtual machine disk images.   

Checking Virtualization Support: Before proceeding with the installation, confirming that your CPU supports hardware virtualization is wise. You can do this by running the following command in your terminal:

egrep -c '(vmx|svm)' /proc/cpuinfo

If the output is 0, your CPU either doesn’t support hardware virtualization or is not enabled in your BIOS/UEFI. If the output is a number greater than 0, it indicates that your CPU supports the necessary virtualization extensions. Remember to enable virtualization in your BIOS/UEFI settings if it’s not already.   

Setting up Virtual Machines: Once the packages are installed, you have a couple of primary ways to set up and manage your virtual machines:

virt-manager (Graphical Tool): For users who prefer a graphical interface, virt-manager provides an intuitive way to create, configure, start, stop, and manage virtual machines. You can define the virtual hardware (CPU, memory, disk, network), install operating systems from ISO images, and monitor the performance of your VMs through a visual dashboard.

virsh (Command-Line Tool): For more advanced users or those who prefer automation, virsh is a powerful command-line utility that interacts with the libvirt daemon. It allows you to perform all the same management tasks as a virt-manager but through text-based commands. virsh is particularly useful for scripting and remote management of KVM hosts.

These steps provide a solid foundation for setting up “kvm virtualization on linux”. You can begin creating and managing your virtual environments by installing the correct packages and utilizing tools like virt-manager or virsh.

KVM vs. Other Virtualization Technologies

While KVM is a robust and popular choice, it’s essential to understand how it stacks up against other prominent virtualization technologies. Here’s a brief comparison:

– KVM vs VMware

VMware offers a wide array of virtualization products, from desktop services like VMware Workstations to enterprise-grade hypervisors like ESXi. VMware boasts a more mature ecosystem with broader features and out-of-the-box management tools. However, VMware’s products are typically proprietary and come with licensing costs. KVM, being open-source and integrated into Linux, offers a cost-effective alternative with strong performance, particularly in Linux environments. While VMware might have a more polished user experience in some areas, KVM’s performance and flexibility are highly competitive.   

– KVM vs VirtualBox

VirtualBox is another open-source virtualization solution primarily aimed at desktop virtualization. It’s known for its ease of use and cross-platform compatibility (running on Windows, macOS, and Linux). While VirtualBox is excellent for individual users and development/testing on local machines, it generally doesn’t match KVM’s performance and scalability for server virtualization and cloud environments. KVM’s closer-to-the-metal architecture gives it an edge in demanding workloads.

– KVM vs Xen

Xen is another established open-source hypervisor, used in cloud environments. Both KVM and Xen belong to Type 1 hypervisors with strong performance. Historically, Xen had an architectural difference that sometimes led to different performance characteristics in specific scenarios. However, modern KVM has evolved significantly and often demonstrates comparable or superior performance in everyday use cases. Both are powerful options; the choice usually comes down to specific requirements and existing infrastructure.

Use Cases of KVM Virtualization

KVM’s versatility and performance have made it a cornerstone in various computing domains:   

– Cloud Computing Platforms (OpenStack, Proxmox)

KVM is a fundamental building block for many open-source cloud platforms like OpenStack and Proxmox VE. These platforms leverage KVM’s capabilities to provide scalable and secure infrastructure-as-a-service (IaaS) solutions. KVM’s performance, security features, and open-source nature make it ideal for building public and private clouds.   

– Development and Testing Environments

Developers and testers frequently utilize KVM to create isolated environments for building, testing, and debugging software. Virtual machines provide clean and reproducible setups, allowing experimentation without affecting the host system. KVM’s performance enables efficient testing of resource-intensive applications.

– Hosting Multiple Linux and Windows VMs

KVM’s ability to support a wide range of guest operating systems, including various Linux distributions and Windows Server and Desktop versions, makes it ideal for server consolidation and running diverse workloads on a single physical machine. This can lead to significant cost savings and improved resource utilization. KVM can handle it efficiently if you need to run multiple Linux-based services or require a Windows environment for specific applications.

KVM Management Tools and Ecosystem

– Command-Line Tools

As an administrator, command-line tools offer robust control over KVM virtualization if you prefer precision and automation. Virsh is the primary tool in this realm. It is a command line interface built on top of the library API. Also, it allows for a comprehensive management of virtual machines, including starting, stopping, pausing, and migrating virtual machines. Its direct interaction with the libvirt daemon ensures efficient and granular control over every aspect of your virtualized environment.

– Graphical Management Tools

For a more intuitive approach to managing KVM virtual machines, graphical management tools like virt-manager offer a user-friendly interface. virt-manager connects to the libvirt daemon and presents a clear overview of your virtual machines, their status, and resource utilization. Through its graphical windows, you can easily create new VMs, configure their virtual hardware (CPU, memory, disk, network), install operating systems via ISO images, and monitor their real-time performance. 

– Proxmox VE

Proxmox Virtual Environment (VE) is a multi-functional and open-source virtualization platform built on KVM and LXC (Linux Containers). This platform provides an easy-to-use web-based management interface that makes creating and managing VMs (KVM-based) and containers easy and straightforward. Proxmox VE includes a variety of features that would be expected in an enterprise virtualization platform, including high availability clustering, live migration, integrated storage management with several options (such as ZFS and Ceph), a built-in firewall, and backup utilities.