Paravirtualization

This is also sometimes referred to as OS assisted virtualization. In this type of technique, the OS code is modified to replace the non-virtualizable instructions with the hypervisor calls. The difference between full virtualization and paravirtualization is that in full virtualization, OS is not aware that it is running on a virtualization layer, and sensitive OS calls are trapped and modified with the help of binary translations.

Paravirtualization can sometimes become overhead as it requires deep OS level code modification.

Building sophisticated binary translation codes are challenging for modern environments, and this is the reason why directly modifying OS code is sometimes considered easy.