VMware virtualization and ESX server is not based on RHEL ! It has it's own kernel called VMkernel, which is 100% percent proprietary. VMkernel runs directly on a hardware as a hypervizor, and there is one special Virtual Machine pre-installed on each ESX server - running on top of this hypervizor. This VM is called Service Console and has special priviledges to manage the hypervizor - inside this VM runs modified version of RHEL - but once again, VMkernel is not based on Linux, the service console runs in the same way as all the other VMs. There is another hypervizor available from VMware which is called ESXi and it has exactly the same VMkernel as ESX has, but there is no service console available on ESXi. This concept is more clear with the new platform called vSphere, where ESX is an essential part of - there is still that service console based on RHEL, but you can see it as a VM with it's own virtual disk (VMDK). In case of any interruption inside the RHEL in service console, it does not affect any other VMs running on the hypervizor.. The hypervizor of ESX is standalone !