What is the difference between the vSphere ESX and ESXi architectures?

VMware ESX and ESXi are both bare-metal hypervisor architectures that install directly on the server hardware. Although neither hypervisor architecture relies on an OS for resource management, the vSphere ESX architecture relied on a Linux operating system, called the Console OS (COS) or service console, to perform two management functions: executing scripts and installing third-party agents for hardware monitoring, backup or systems management.

In the vSphere ESXi architecture, the service console has been removed. The smaller code base of vSphere ESXi represents a smaller “attack surface” and less code to patch, improving reliability and security.