In this days it's avaible Windows 2012 Server R2 preview.
If you are interested to download the relative .vhd images immediately running you can go to the following link:
http://technet.microsoft.com/it-it/evalcenter/dn205286.aspx?CR_CC=200142594
If you want to view a very simple tutorial to enable it on HyperV on a Server or Windows 8 here is the link:
http://blogs.technet.com/b/italy/archive/2013/08/01/come-installare-windows-server-2012-r2-preview-in-hyper-v.aspx