A ( ) is a "completely isolated guest operating system installation within a normal host operating system". [ 1 ] Modern virtual machines are implemented with either software emulation or hardware virtualization . In most cases, both are implemented together. [ edit ] VM Definitions A virtual machine (VM) is a software implementation of a machine (i.e. a computer) that executes programs like a physical machine. Virtual machines are separated into two major categories, based on their use and degree of correspondence to any real machine.