Here some results from testing I did in August 2009 on KVM with the three different disk image drivers. First a single disk system running Ubuntu x64 9.04:
From the vmware blog:
A 1vCPU Xeon X5500 series based Exchange Server VM can support 50% more users per core than a 2vCPU VM based on previous generation processors while maintaining the same level of performance in terms of Sendmail latency. This is accomplished while the VM’s CPU utilization remains below 50%, allowing plenty of capacity for peaks in workload and making an FT VM practical for use with Exchange Server 2007.