Unix has several ways to set up virtualization, where a physical machine has one or more “Virtual Machines” running on it, each of which assumes they are on a bare metal machine of their own.
Linux Virtual Machine Managers
Linux virtual machines can be managed from the command line, but each flavor of virtualization (Xen, KVM, etc…) has its own syntax. However, there are managers which sit on top of one or more of these which provide more integrated control, in many cases handling more than one virtualization technology at a time. So, a virtualization manager may handle Xen and KVM hypervisor, for example.