On a fresh install of Windows Server 2012 R2 in a Hyper-V host role, with four VMs running but idle and nothing going on in any of the VMs or on the host, I get a periodic CPU spike on a regular periodic basis (about every 12 seconds). The process that's generating the spike, per Task Manager on the host machine, is vmms.exe. The spike are pretty severe, lasting a few seconds and using close 80-100% of one core. What would be causing this? How can I diagnose it (i.e. other than knowing that it's vmms.exe)?
Periodic CPU spikes from VMMS indicate that you're running some monitoring tool that is polling information about the VMs. This could be the Hyper-V manager, System Center Virtual Machine Manager, etc.