Xen/KVM market share probably dwarfs VMware if you look at total systems. It's what a lot of the large cloud providers use (AWS/GCE at least). I assume Azure uses HyperV, and also that a lot of shops are using it because it's the supported MS product. Our company has VMware and HyperV deployments for just that reason (Office IT has their own MS cluster, systems uses a VMware cluster we admin. Yes, it's silly, but does illustrate a point).