VMM (Virtual Machine Monitor)

Software that is responsible for virtualizing the CPUs

virtualization software layer managing hardware requests from a virtual machine, passing them to a hypervisor, and simulating answers from real hardware.

The virtualization software layer managing hardware requests from a guest OS, simulating answers from a real hardware. Used as Hypervisor synonymous.

Software that runs in a layer between a hypervisor or host operating system and one or more virtual machines that provides the virtual machine abstraction to the guest operating systems. With full virtualization, the virtual machine monitor exports a virtual machine abstraction identical to a physical machine, so that standard operating systems (e.g., Windows 2000, Windows Server 2003, Linux, etc.) can run just as they would on physical hardware.

the software that allows multiple virtual machines to be multiplexed on a single physical machine.

A layer of software or firmware/microcode that sits between one or more operating systems and a server. It presents each hosted operating environment with the impression that it has it's own dedicated server, although in reality they all share the same physical server. [Also Virtual Machine Manager]