Alright, since we already know at a very high level what KVM is (a hypervisor), let’s delve into the different components that it is comprised of. KVM is implemented as a kernel module that can be loaded to transform Linux into a Virtual Machine Manager (VMM). As Linux already had all of the tools and mechanisms needed to house several VMs, the developers just needed to add a few components to support virtualization. Each process in a standard Linux environment runs in one of two modes: user-mode or kernel-mode. The advent of KVM introduced a third: guest-mode, which relies on a virtualization capable CPU. With guest-mode, certain instruction sets can be “trapped”, so to speak. In KVM, each VM is implemented as a process, which relies on it’s scheduler for the assignment of computing power to the virtual machines; memory is allocated via the Linux memory allocator.
The two components that make up KVM are: /dev/kvm and QEMU (Wow, it's that simple!). Once the KVM kernel module is loaded (this is not enough to run virtual machines on all by it’s lonesome), the /dev/kvm device node appears in the file system. The hypervisor can be controlled through this interface via a set of ioctls - system calls that execute operations to create new and assign resources to VMs. KVM also used a generic emulator Quick Emulator, better known as QEMU to present hardware to the VMs. For each virtual machine, a separate QEMU process is started in user-mode, and certain emulated devices are virtually attached. Read and write I/O operations from the VM are intercepted by the hypervisor and redirected to the associated QEMU process for that specific guest.
“Since a virtual machine is simply a process, all of the standard Linux process management tools apply: one can destroy, pause, and resume a virtual machine with the kill command (or even using Ctrl-C and similar keyboard shortcuts) and view resource usage with top. Permissions are handled by the normal Linux method: the virtual machine belongs to the user who started it (which need not be root; all that is required is access to /dev/kvm), and all accesses are verified by the kernel.”
Sources: http://www.linuxinsight.com/files/kvm_whitepaper.pdf
http://www.cs.hs-rm.de/~linn/fachsem0910/hirt/KVM.pdf
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