Wondering how all the I/O requests by virtual machines are handled in Hyper-V? Hyper-V has a storage IO balancer which prevents one busy VM from starving another VM. The Performance Tuning Guidelines for Windows Server 2008 at http://www.microsoft.com/whdc/system/sysperf/Perf_tun_srv.mspx provides the following detail:
I/O Balancer Controls

The virtualization stack balances storage I/O streams from different VMs so that each VM has similar I/O response times when the system’s I/O bandwidth is saturated. The following registry keys can be used to adjust the balancing algorithm, but the virtualization stack tries to fully use the I/O device’s throughput while providing reasonable balance. The first path should be used for storage scenarios, and the second path should be used for networking scenarios[…]

Full Article