Testing using a workload that mimics real world SQL Server workloads better than ever before, suggests that ESX today has a VM density ratio well in excess of 1.5:1.
VMware’s ESXi 4.1 continues to lead the pack, delivering a density advantage of at least 2:1 and up to almost 3:1 versus Hyper-V R2 and between 1.7:1 and 2.3:1 vs. KVM.
XenServer has closed the density gap in terms of number of concurrent VMs that can be run on a given host, coming to par with ESXi, but that this comes with a significant and unacceptable performance penalty. XenServer consistently delivers far less performance across the board (penalty ranging from 25% to 69%), and in our view gives ESXi as much as a 2:1 density advantage over XenServer, once we consider the ability of the hypervisor to access the full performance of the underlying hardware.