A couple of weeks ago I posted about the fact that Windows Server 2012 R2 is not supported on top of Windows Server 2008 R2. 

This generated a lot of feedback about the need to improve our documentation for which operating systems are supported on which versions of Hyper-V.  So the Hyper-V documentation team has been busy at work.  Specifically they have pulled the guest OS support information out of the current Hyper-V overview and split it into 2 topics:

They have also added a note about Windows Server 2012 being the last version of Windows that will be supported as a guest operating system on 2008 R2 to this topic: http://technet.microsoft.com/library/cc794868.aspx

Cheers,
Ben