If you run Windows 2012 guests on RHEV or Ovirt (or probably any KVM based hypervisor) and notice performance is lower than expected, take a look at the power management in the BIOS of your servers. After lot's of test benchmarks with mixed results with Windows 2012 guests on RHEV with Dell m620 blades, we finally discovered our performance problems were caused by the system profile "Performance Per Watt (DAPC-System)" in the BIOS. Changing this profile to "Performance Per Watt (OS)" almost doubled the performance for some CPU intensive tasks in our case. Power usage of a (mostly idle) dual socket blade increases with 30-50 Watts when you change this. Used servers in production are using already more power and I guess the difference will be less there.
We only had this issue with Windows 2012 guests, for Linux guests the performance is always close to bare metal performance.
We spent a lot of time on this and also opened a case at Red Hat (and sent them the solution ;-)). As far as I know, this problem was nowhere documented till now... except... in the VMware knowledge base.
Update 24/11/2015
Red Hat confirms the issue in their KB