What can affect the VM? Network utilization, CPU on host, drive subsystem, drive controller, older/buggy drivers, VM guest OS, VM tools installed in the guest, drive filesystem, fragmentation, host utilization, memory use and availability, drive speed, RAID type, cache on RAID controller, number of host CPU's, virtual CPU's, day of the week, noise in the server room affecting drive vibration, I/O due to backing up systems or migrating them, virtual switch load, number of VM's on the machine...what doesn't have some affect on VM performance?
What exactly are you trying to do? Is there a specific instance where you're facing a particular bottleneck you're trying to monitor or discover? Otherwise this question is rather vague. What works for one setup won't necessarily be acceptable in another.