Poor gaming performance with hyper-v installed in windows 8
Solution 1:
If I were you, I would consider using vmware or virtualbox.
I use virtualbox myself and don't have any performance issues when playing games. I have even left a vm running and not noticed a difference. Granted I have lots of RAM and a good processor.
Solution 2:
As far as I can see there should only be a minimal performance reduction if you enable Hyper-V. If found this thread in the MSFT forum.
This is because of that you Windows 8 is then running also on Hyper-V as you can see here.