KVM

KVM stands for Kernel-based virtual machine. It provides all system call methods that the userspace VMM applications can use to enable virtualization on the host.

KVM provides two main functions. One is to control the CPU to run virtual machines threads. Other is to provide memory mapping for the virtual machines.

why kernel based?

KVM is a linux kernel module that's shipped together with the kernel itself. This converts the host into a hypervisor.

CPU also must support

It's not enough for the host to be running Linux kernel with KVM module.

This is because the CPU has the concept of rings. In case of virtual machines, every VM also has its own kernel. Hence, the CPU must differentiate between the ring 0 of the host kernel itself and the guest kernel itself.