T he distinction between Type 1 hypervisors (being minimal OSes designed only to host VMs) and those of Type 2 (which run VMs inside a regular operating system) can get a little muddy. KVM, which userspace programs like VirtualBox and QEMU can use, might appear to be categorically of the second type. But that’s not really correct. The KVM module actually transforms the whole Linux kernel into a ‘bare metal’ hypervisor, and any kernels you load in a VM sidestep the rest of your OS. Your userspace doesn’t become transformed into a VM (and indeed is untouched by the whole process), but it does add some convenient abstractions (networking and such) so that you can take to guests through it. Hyper-V does much the same thing in Windows.
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XCP-NG IN A NUTSHELL “XCP-ng is a next-generation Type 1 hypervisor, based on XenServer. It’s open source and makes easy work of hosting a truckload of VMs.”