Why use Hyper-V for Linux VM's? Put simply, for compatability. We purchased Windows Datacenter for 3 VM Hosts. Why Datacenter you may ask? Because DataCenter is licensed per CPU socket, so we can run as many Windows installs as we can on each physical system without having to purchase additional Licenses. We also use Linux, CentOS mostly, and having support for CentOS in HyperV means that we don't have to purchase or run other Hypervisors to support hose installs. With a single hypervisor platform we can do all those cool high availability things, and we don't have to train everyone on multiple hypervisors.
I was originally a VMware guy, and I like XenServer, but it comes down to simplicity and cost considerations. If I want all the benefits of the setup I have, but on XenServer or VMware, I have to purchase a full on Hypervisor setup, like vCenter, and then I still have to buy copies of Windows for each of my MS vms. That is just too much damn money, when I can get a much bigger bang-for-my buck with a few copies of Windows Datacenter!
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