How KVM works & its architecture

KVM (Kernel-based Virtual Machine) is a virtualization technology built into the Linux kernel that allows it to function as a hypervisor. KVM enables running multiple virtual machines (VMs) on a host system, providing full isolation between VMs. The architecture of KVM is tightly integrated with the Linux kernel, offering both performance and scalability.

How KVM Works:

1. Hypervisor Mode (Virtualization Support): KVM utilizes hardware virtualization extensions (like Intel VT-x or AMD-V) available in modern processors. These extensions allow the host CPU to run in "host" mode (bare-metal OS) and "guest" mode (virtualized OS) efficiently. The CPU's hardware support enables virtual machines to execute code directly on the hardware, rather than relying on software emulation.


2. Kernel Module: The KVM implementation consists of two parts:
   
   
   
   • kvm.ko: This is the core kernel module that provides the virtualization capabilities.
   
   
   • kvm-intel.ko or kvm-amd.ko: These are processor-specific modules that enable virtualization support for Intel or AMD processors, respectively.
   
   
   
   These kernel modules interact with the hardware virtualization extensions and provide the functionality needed to create, manage, and isolate VMs.


3. QEMU: QEMU (Quick Emulator) is used as the user-space component of KVM. It acts as a virtual machine monitor (VMM) and provides device emulation, managing the virtualized devices (e.g., virtual CPUs, memory, storage, and network interfaces) for the VMs. While KVM provides the low-level virtualization, QEMU handles the virtual hardware and system management.


4. Libvirt: Libvirt is an API, daemon, and management tool used for interacting with KVM. It provides a user-friendly interface to configure and manage virtual machines, networks, and storage devices. Libvirt is often used in conjunction with other tools like virsh (command-line interface) or graphical tools like virt-manager.

KVM Architecture:

The KVM architecture can be broken down into several layers:

1. Host Layer: The host system is the physical machine where KVM operates. It runs a Linux-based OS with KVM modules loaded into the kernel. The host provides resources (CPU, memory, storage) for the guest VMs.


2. Virtual Machine Layer: Each virtual machine (VM) is a separate entity that runs its own operating system (the "guest"). VMs have their own virtualized resources (CPU, memory, network, and storage). They operate in an isolated environment but can access resources of the host via KVM and QEMU.


3. Hypervisor Layer: KVM itself functions as the hypervisor. It allows the kernel to manage the virtual machines, allocate CPU time, memory, and I/O operations. It works by enabling the CPU to switch between guest and host mode based on the need for execution. KVM uses hardware acceleration for the most efficient execution, but it can also fall back on software-based virtualization if necessary.


4. Device Emulation Layer: QEMU provides the emulation of virtual devices that each VM uses. For example, it handles virtual hard drives, network interfaces, and graphical consoles. QEMU also provides services like memory ballooning and snapshots for the VMs.


5. Management Layer:
   
   
   
   • Libvirt: As mentioned, Libvirt interacts with the KVM and QEMU stack to manage virtual machines, create new VMs, configure virtual hardware, and start/stop the VMs. It abstracts some of the complexity of managing VMs and offers a convenient API to interact with them.
   
   
   • Virtual Machine Management Tools: Tools like virt-manager (GUI-based) and virsh (CLI-based) are used for managing virtual machines through Libvirt.
   
   
   
   

KVM Architecture Diagram:

Key Features of KVM:

1. Full Virtualization: KVM supports full virtualization, meaning the guest OS doesn't need to be aware that it's running on a virtualized machine. This is achieved through hardware extensions.


2. Isolation: KVM provides strong isolation between VMs, allowing each to run independently without interfering with others. This is particularly important for multi-tenant environments.


3. Live Migration: KVM supports live migration, which allows moving running VMs from one physical machine to another with minimal downtime.


4. Resource Management: KVM and QEMU allow dynamic resource management, including CPU pinning, memory ballooning, and CPU/IO resource allocation to ensure VMs are optimally running.


5. Scalability: KVM can scale from small setups to large data centers, supporting a high number of VMs per host and providing the flexibility for growth.


6. Open Source: Being part of the Linux kernel, KVM is open-source software, which makes it free to use and modify. It has a large community for support and contributions.

Conclusion:

KVM VPS works by leveraging hardware virtualization features in modern CPUs, using the Linux kernel to manage virtual machines, and QEMU for device emulation. Its architecture is highly efficient and scalable, making it suitable for both small-scale setups and large data centers. Through tools like Libvirt, managing virtual machines becomes more straightforward, and the system integrates well with other management platforms.