What is a Virtual Machine

A virtual machine (VM) is a software-based representation of a physical computer that can run its own operating system and applications, all isolated from the host system and other virtual environments. KVM (Kernel-based Virtual Machine) is a widely used Linux-based hypervisor that enables Linux to act as a type-1 hypervisor, transforming the host system into a platform that can manage multiple virtual machines. Because KVM is built into the Linux kernel, it is a robust, efficient, and open-source solution that supports various operating systems, including Linux and Windows, on a single host.

KVM has become a popular choice in data centers and enterprise environments due to its scalability, cost-effectiveness, and ability to provide full OS-level isolation. In a KVM setup, each VM is essentially a collection of files that define its components and resources. These files form the foundation of the VM before it starts, and they come to life only when the VM is booted up.

The Files that Define a KVM-Based Virtual Machine

Before a KVM virtual machine begins running, it exists as a set of files stored on disk. Each file plays a specific role in defining the VM's configuration, storage, and behavior:

Virtual Disk Files: KVM VMs typically use QCOW2 (QEMU Copy-On-Write) or RAW file formats for virtual disk storage. These files contain the guest OS, applications, and data, acting as the VM's virtual hard drive. QCOW2 files support advanced features like snapshots, which allow users to capture and revert to previous states of the VM, making them a popular choice.
XML Configuration Files: Every KVM VM has an associated XML file, often managed by libvirt, that contains key settings such as CPU allocation, memory, storage, and network configuration. This file defines the virtual hardware specifications, allowing the hypervisor to allocate resources appropriately.
Snapshot and Backup Files: Snapshots capture the VM's state at a specific moment, including memory, disk, and CPU settings, enabling rollback to a previous state if needed. These files are useful for testing, backups, or any scenario where reverting to a prior configuration is beneficial.
Log Files: Log files store information about the VM's operations, including boot activity, shutdown events, and any errors encountered. These files are valuable for troubleshooting and performance monitoring.
Metadata and Supporting Files: Metadata files may contain additional information about the VM's configuration, particularly when using management tools or in clustered environments where network and storage configurations need to be coordinated across multiple hosts.

These files collectively define the structure and configuration of a KVM virtual machine. However, the VM remains static until runtime when the hypervisor launches it, initiating a sequence that brings the VM's virtualized environment to life.

KVM vs. Containers

Although both VMs and containers are virtualization technologies, their underlying structures and resource requirements differ significantly. KVM VMs include a complete OS stack, which means each VM has its own kernel, OS, and applications. This requires a dedicated virtual disk file (like QCOW2) to store all system files and data, resulting in higher resource usage and setup complexity compared to containers.

Containers, by contrast, share the host OS kernel and operate without a full OS stack. This lightweight structure means containers require fewer files and start much faster than VMs. However, they are limited to running the same OS kernel as the host (e.g., Linux), and they provide less isolation since they rely on kernel namespaces and control groups for separation, rather than a full hypervisor layer.

For use cases where security and isolation are top priorities or when different OS types are required on the same host, KVM virtual machines are a better fit. Containers are better suited to lightweight, microservices architectures where speed and efficiency are critical, and isolation needs are less stringent.

What Happens at Runtime: Booting a KVM Virtual Machine

When a KVM-based VM is started, it undergoes a process similar to a physical machine's boot sequence. The KVM module loads the virtualized kernel, and the hypervisor manages interactions between the VM and the host hardware, creating a seamless experience for the guest OS.

Virtualized Kernel: At runtime, the VM's kernel runs in a virtualized environment facilitated by KVM, which uses hardware virtualization extensions (such as Intel VT-x or AMD-V) to directly access the CPU. This provides a near-native performance experience while keeping the VM's operations isolated from other VMs and the host.
Initialization and Init Process: Once the kernel is loaded, the VM's OS initiates its init process. This sequence is responsible for starting essential services, loading system modules, and setting up the operating environment. Just as with a physical machine, the init process ensures that the VM is prepared to run applications.
Resource Management by KVM: During runtime, KVM uses qemu-kvm to manage resource requests, such as CPU cycles, memory allocation, and input/output operations. The hypervisor dynamically allocates and monitors resources to ensure optimal performance for each VM while preventing interference between VMs on the same host.
Networking and Storage: Networking within a KVM VM is handled by virtual NICs (network interface cards) and virtual bridges, enabling the VM to communicate with the host or other VMs as if it had its own physical network card. Storage is managed through the virtual disk files (QCOW2 or RAW) created at setup, and KVM treats these files as virtual hard drives, closely emulating physical storage.

Differences from Container Runtime

Unlike VMs, containers skip the full OS boot and initialization process. Instead, they leverage the host OS kernel directly, allowing for faster startup times. This difference is a primary reason containers are preferred in environments prioritizing speed and resource efficiency over strict isolation.