I've spent time recently looking into the performance of visualized SQL instances on SAN storage. The systems present a myriad of possibilities with regards to how exposed drives are configured i.e. physical disks, LUNs and virtual drives. Smart SAN solutions can move data around according to frequency of use, placing the most accessed data on the fastest storage. Virtual Hosts can reallocate resources amongst guests e.g. if a failure occurs or for a data processing window. Resources available to a VM can therefore change, maybe even without the guest OS being aware.
Anyway, Paul Randal has a script for looking at your I/O throughput as seen by SQL Server.
It's a lot quicker than using Perfmon (Performance Monitor).
How to examine IO subsystem latencies from within SQL Server
