The virtualization component that manages the guest OSs on a host and controls the flow of instructions between the guest OSs and the physical hardware.
NIST SP 800-125 under Hypervisor
A software built using a specialized kernel of an OS, along with supporting kernel modules that provides isolation for various execution stacks represented by Virtual Machines (see below).
NIST SP 800-125A under Hypervisor
NIST SP 800-125 under Virtual machine monitor (VMM)