MED-V is a virtualization technology that was enabled by Virtual PC on Windows 7. It had the advantage of presenting applications installed in a Windows XP virtual machine directly in Windows 7. Nice, but not widely used.
In Windows 8, Virtual PC has been replaced in Windows 8 by Client Hyper-V. As a consequence MED-V is not supported on Windows 8. The Windows XP Mode included as part of Windows 7 is also unsupported due to the loss of Virtual PC.
An announcement is here: http://windowsteamblog.com/windows/b/business/archive/2012/06/12/mdop-news-at-teched-north-america-2012.aspx