VMware's ESX hypervisor and its vCenter control suite are thought to be operating in more than 80 percent of all enterprise IT systems. And some IT people believe that number is too conservative.

Microsoft's Hyper-V, still getting its bearings in the market, is growing in use but its market share is still mired in the single digits. That leaves 10 to 15 percent of all the rest deploying other virtual systems middleware, mainly the open-source XenServer and KVM hypervisors that come bundled in most Linux distributions.