Virtualisation has gone through four main phases:

• full software emulation - every instruction is emulated - very, very slow. (Bochs)
• software virtualisation - instructions are checked but most run natively. (VMWare server)
• paravirtualisation - the guest operating systems know they are virtualised and are tweaked appropriately (Xen)
• hardware assisted virtualisation - the CPU provides assistance to the above methods (KVM)

The programs above are only very roughly categorised, for example virtio provides paravirtualisation to KVM.