ESX runs on bare metal (without running an operating system) unlike other VMware products. It includes its own kernel: A Linux kernel is started first, and is then used to load a variety of specialized virtualization components, including ESX, which is otherwise known as the vmkernel component. The Linux kernel is the primary virtual machine; it is invoked by the service console. At normal run-time, the vmkernel is running on the bare computer, and the Linux-based service console runs as the first virtual machine. VMWare dropped development of ESX at version 4.1, and now uses ESXi, which does not include a Linux kernel.
The vmkernel is a microkernel with three interfaces: hardware, guest systems, and the service console (Console OS).