What is KVM?
KVM, which means Kernel-based Virtual Machine, is a virtualization solution directly integrated into the Linux core. Since its inclusion in the kernel 2.6.20, KVM allows to convert a standard Linux server into a type 1 hyperviewer capable of running multiple virtual machines with a performance close to that of the bare metal hardware. Unlike hosted virtualization solutions, KVM takes advantage of hardware extensions such as Intel VT-x and AMD-V to provide superior insulation and efficiency. Each virtual machine is implemented as a regular Linux process, which facilitates its monitoring, debugging and management through the usual system tools.
Main components of KVM
- The kvm.ko kernel module, which provides the virtualization interface to the kernel.
- QEMU, the device emulator that provides support for CPU, memory, storage and input / output devices.
- Libvirt, a management layer that offers a unified API to create, modify and control virtual domains.
- Command line tools such as virsh and virt-manager that facilitate graphical and text-based administration.
Advantages of using KVM in Linux
KVM stands out for its performance, security and zero cost. Being integrated into the kernel, it benefits from the security updates and performance improvements that Linux receives. The virtualization overload is minimal, typically less than 5% in CPU workloads and intensive memory. In addition, KVM inherits the Linux permissions model, which allows to isolate virtual machines through users and groups, and to apply SELinux or AppArmor policies. Another strong point is its scalability: from a laptop with two cores to rack servers with hundreds of threads, KVM is adapted without costly licenses.
System requirements
- Processor with virtualization extensions (Intel VT-x or AMD-V) activated in the BIOS / UEFI.
- Kernel Linux version 3.10 or higher, although the latest versions of the 5.x or 6.xseries are recommended.
- At least 2 GB of RAM free for the host, plus the memory assigned to each virtual machine.
- Sufficient disk space, preferably in a file system that supports simultaneous writing operations such as ext4, XFS or btrfs.
- Access to root privileges or to a user with sudo permissions to load modules and manage VM.
Step-by-step installation
- Check availability of hardware extensions with command
lscpu | grep Virtualization; must show VT-x or AMD-V. - Install the necessary packages. In Debian / Ubuntu based distributions:
sudo apt update && sudo apt install qemu-kvm libvirt-daemon-system libvirt-clients bridge-utils virt-manager. - In RHEL / CentOS / Fedora systems:
sudo dnf install @virtualizationorsudo yum groupinstall virtualization. - Add user to group
libvirtand optionally to the groupkvmto allow unsuted access:sudo usermod -aG libvirt $USER && sudo usermod -aG kvm $USER. - Reset the session or close and relog so that the group changes take effect.
- Check that the module is loaded:
lsmod | grep kvmshould show kvm _ intel or kvm _ amd. - Start and enable the libvirtd service:
sudo systemctl enable --now libvirtd. - Validate the installation by running
virsh list --all; should show an empty list of domains, indicating that the hyperviewer is ready.
Network configuration and storage
For virtual machines to communicate with the outside world and with each other, KVM offers several network modes. The default NAT mode allows VM to access the Internet by translating host addresses, but does not give them an IP accessible from the local network. When direct access is needed, a bridge is created that connects the physical interface of the host to a virtual interface of the VM, allowing it to obtain an IP of the same segment as the host. In terms of storage, KVM supports qCow2-format images, which support snapshots and compression, or raw-format disks for maximum performance. libvirt storage tools allow you to manage discs locally, in LVM, in distributed file systems such as Ceph or in NFS devices, offering flexibility for development, testing and production environments.
Good practice and optimization
- Assign only the necessary cores and memory to each VM; overuse of resources can degrade the performance of the host and other virtual machines.
- Use qCow2 discs with written cache in the host to balance performance and security against energy failures.
- Enable the fusion of transparent pages (KSM) when running multiple VM with identical operating systems, which reduces memory consumption by sharing identical pages.
- Keep the kernel and virtualization packages up to date; new versions include improvements in the CPU programmer and in the management of interruptions.
- Monitor the use of CPU, E / S of disk and network traffic with tools like virt-top, netdata or Prometheus along with libvirt exporters.
- Consider the use of pinning CPU and hugepages for high performance workloads, such as databases or intensive calculation applications.
Conclusion
KVM has been consolidated as the default virtualization option in Linux environments thanks to its deep integration with the core, its low cost and its ability to deliver a performance close to the physical hardware. From small developers that test applications on your laptop to large private cloud providers that run thousands of workloads, KVM provides the flexibility, security and scalability needed to meet current infrastructure requirements. By following the installation steps, properly configuring the network and storage, and applying the good practices described above, any system manager can deploy a robust and efficient Linux-based virtualization platform.
