Can Windows Server 2012 run as a guest VM under Server 2008 R2 Hyper-V Host?
According to this MS KB article it is possible but may thow some warning messages here and there.
You could try this:
An update that enables Windows 8 or Windows Server 2012 to be hosted in a Hyper-V virtual machine on Windows Server 2008 R2
http://support.microsoft.com/kb/2526776
Edit: hotfix 2526776 has been superseded by a newer hotfix:
You cannot run a Windows 8-based or Windows Server 2012-based virtual machine in Windows Server 2008 R2
http://support.microsoft.com/kb/2744129