A hypervisor that delivers full virtualization, whether or not it uses hardware-assisted virtualization, does not require any changes to the guest OS, which is not always the case for paravirtualization. If a Linux distribution includes paravirt operations extensions in its kernel, it supports paravirtualization. Otherwise, it needs to be modified. In fact, any OS that does not support paravirtualization must be modified, which leaves out proprietary OSes, such as Windows.