The Hypervisor, which is also called virtual machine monitor (VMM), is the main building block of virtualization technology. It allows multiple operating systems to run concurrently on a host machine. The hypervisor presents the guest operating systems with a virtual platform and monitors the execution of the guest operating systems. In that way, multiple operating systems, including multiple instances of the same operating system, can share hardware resources. There are two types of hypervisors, illustrated in figure below and categorized as follows: