KVM has become one of the most widely used virtualization technologies today, and it has taken on many different forms by companies or organizations that have modified the code, including IBM and Red Hat. It is an open-source alternative to proprietary virtualization technologies such as ESXi offered by VMware and Microsoft.
KVM is a basically unique hypervisor. The KVM developers, instead of creating major portions of an operating system kernel themselves, as other hypervisors have done, devised a method that turned the Linux kernel itself into a hypervisor.
This was achieved through a minimally intrusive method by developing KVM as kernel module. Integrating the hypervisor capabilities into a host Linux kernel as a loadable module can simplify management and improve performance in virtualized environments. This probably was the main reason for developers to add KVM to the Linux kernel.