In computing, a hypervisor or virtual machine manager (VMM) is a piece of computer software, firmware or hardware that creates and runs virtual machines.
A computer on which a hypervisor is running one or more virtual machines is a host machine. Each of those virtual machines is called a guest machine. The hypervisor presents to the guest operating systems a virtual operating platform and manages the execution of the guest operating systems. Multiple instances of a variety of operating systems may share the virtualized hardware resources.
Read more about Hypervisor: History, Classification, Mainframe Origins, UNIX and Linux Servers, PCs and Desktop Systems, Embedded Systems, Security Implications