Hyper-V, codenamed Viridian and formerly known as Windows Server Virtualization, is a native hypervisor that enables platform virtualization on x86-64 systems. A beta version of Hyper-V was shipped with certain x86-64 editions of Windows Server 2008, and a finalized version (automatically updated through Windows Update) was released on June 26, 2008. Hyper-V has since been released in a free stand-alone version, and has been upgraded to Release 2 (R2) status. It was updated in Windows Server 2012.
Read more about Hyper-V: Versions and Variants, Architecture, System Requirements and Specifications, Supported Guests, VHD Compatibility With Virtual Server 2005 and Virtual PC 2004/2007, Windows Server 2012