I have been asked this question a couple of times recently. The odd thing, to me anyway, is that most of the people who have asked me have stated:
I read this article: http://msdn.microsoft.com/en-us/library/cc794868(v=ws.10).aspx about supported guest operating systems for Windows Server 2008 R2, and did not see Windows Server 2012 R2 listed. Is it supported?
The answer is – no. That is why it is not listed.
Generally speaking, we make sure that the we support the release immediately after the host operating system (i.e. 2012 on 2008 R2, 2012 R2 on 2012, etc…). But we do not go beyond that.
If you really want to run Windows Server 2012 R2 in a virtual machine – I highly recommend that you upgrade your host first. Besides which, there are so many great new features in Hyper-V in Windows Server 2012 and 2012 R2 that you should want to take advantage of!