I recently switched from Hyper-v Core 2008R2 to Hyper-V Core 2012, and ever since I have problems with a Windows 8 VM.
The server runs a Windows Server 2008R2 standard VM and a Windows 8 Pro VM. Both have sufficient resources availalbe (it's run on a Core i5 2500 with 8GB of RAM), and I have not seen either of them requesting more resources (they are both allowed to request more RAM).
Since the switch to 2012, the Windows 8 VM has been slowing down when not in use. It goes to the point that if I connect to it, it takes a minute before it responsive. It even takes a while before I get see the desktop.
The Windows Server 2008R2 on the other hand still runs as smooth as before.
This would not be such a big issue if the W8 machine did not have applications running that need access to the network an copy large amounts of data. The network connection is frequently dropped while in "slow" state, and will continue to do so until I connect with remote desktop or the hyper-v manager.
It almost looks like some sort of power saving mode. I don't mind this, but it should be able to accomplish automated tasks.