This article, Virtualization wars: VMware vs. Hyper-V vs. XenServer vs. KVM, is a good write up about what is happening in the Virtualization wars.
"This flurry of improvements is in addition to progress Hyper-V has been making against ESX in licenses issued. Hyper-V grew 62% last year compared to ESX's 21% growth and Citrix's 25%, according to IDC. Separately, Gartner projects that by next year Hyper-V will account for 27% of the market, up from 11% two years ago. Within that projected 27%, Gartner says Microsoft will have captured 85% of all businesses with less than 1,000 employees that use virtual servers."
It looks like Microsoft is making some inroads for MS shops. The trick is talking about market share when a lot of virtualization solutions are free. Xen usage is bigger than just Citrix market share. What is the total mindshare for Xen? It seems most public cloud hosting environments pick Xen and then KVM. (And then ESXi).
I find market share hard to discuss. But, it is hard to gauge mindshare. Amazon WS uses a highly customized XenServer. I'd say that have cornered the market on cloud computing mindshare.
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