Which hypervisor type runs directly on hardware without a host OS?

Type 1 hypervisors, also known as "bare-metal" hypervisors, run directly on the physical hardware of a server. This direct access allows them to manage hardware resources efficiently and securely, offering high performance and isolation for virtual machines (VMs). Examples include VMware ESXi, Microsoft Hyper-V, and Citrix XenServer. Type 2 hypervisors, in contrast, run as an application on top of a host operating system (e.g., VMware Workstation, VirtualBox). This introduces an additional layer of abstraction, which can impact performance and resource utilization compared to Type 1. Type 3 and Type 12 are not standard classifications for hypervisor types.