VMware ESX is an enterpise-level virtualization product; it is bare-metal embedded hypervisor (along with ESXi), loaded prior to any actual operating system. Up to version 4.1, a Linux kernel is started first, which runs all the required VMWare services, including the vmkernel component.
The vmkernel has three interfaces to outer world: hardware, guest systems and a service console (Console OS). CPU and RAM are accessed directly; other hardware is available through modules (many modules are derived from Linux kernel). Guest systems are accessing simulated hardware so that they run unmodified (although VMWare drivers can be used to speed up access to the hardware).
Related: WMvare monitoring