Thanks for the comment, but I don’t think you are right in this – a hypervisor is not an operating system: both ESXi and Xen are software stacks that run on top of an operating system – the commercial products actually package a small Linux distribution and install that “underneath” the hypervisor.

Technically term “hypervisor” is currently used to mean the software that interacts with the CPU to run another operating system in “protected mode” that is protected from the host operating system (whether it is a prepackaged light weight Linux or an operating system that is used to run other things such as a graphical desktop), and in that sense both VirtualBox and HyperV are fully qualified hypervisor. You can say that commercially you prefer to use the term “hypervisor” to only mean products that run on top of a packaged operating system and do not let you access the host operating system as a general purpose computing platform, but I don’t think that is technically correct.