(1) See
virtual machine monitor.
(2) A desktop user interface for managing Xen, KVM and QEMU virtual machines. It is used to install and clone VMs and show their current status. Virtual Machine Manager also provides a VNC remote control connection to the guest OS.
(3) The underlying operating system component of Windows. It manages the computer's memory and virtual machines. The services in the Virtual Machine Manager are not directly called by Windows applications, but are accessible by virtual device drivers (VxDs).