The following tweaks help you tune performance with regards to CPUs in Hyper-V.
There are two things that are absolute musts to get any performance out of Hyper-V. Enable integration services on both the Hyper-V hypervisor (Windows server) and on the virtual machines (Windows 7, 8). This will ensure that the necessary code (the drivers) will be installed into the virtual machine (Windows 7, 8) to allow it take advantage of Hyper-V Enlightened I/O.
Don't run any other roles on the Windows server. If you need other roles such as DHCP or DNS, run those inside of a virtual machine. Yes, you can run Windows server on a virtual machine on Hyper-V – which runs on Windows server.