VMkernel

VMkernel is the core operating system kernel of VMware's ESXi hypervisor. It runs directly on server hardware (bare-metal) and provides the virtualization layer that enables multiple virtual machines to share CPU, memory, storage, and networking resources. As a purpose-built kernel, VMkernel implements the essential functionality required to run and isolate virtual machines while delivering performance and security. The kernel is modular, with components loaded as VMware installation bundles (VIBs) that supply device drivers and related functionality.

VMkernel handles CPU scheduling, memory management, and I/O virtualization. It includes a virtualization-aware scheduler to fairly allocate CPU cycles to virtual machines, and memory management features such as reclamation techniques to cope with pressure, including ballooning and, in some configurations, compression. It also manages I/O paths and interrupt routing for devices, enabling efficient interaction between VMs and hardware.

Networking and storage are integrated into VMkernel through a dedicated network stack and I/O subsystems. VMkernel supports VMkernel adapters (vmknic) connected to virtual switches (vSwitch) or distributed switches, facilitating traffic for management, vMotion, fault tolerance, and vSAN. Storage traffic can be carried over protocols such as iSCSI and NFS, and Fibre Channel via host bus adapters, with drivers delivered via VIBs. VMkernel also provides offload and hardware acceleration interfaces where available.

In the ESXi architecture, VMkernel is the central component alongside management agents and user-space services. It enforces resource allocation policies, security boundaries, and fault isolation, and exposes interfaces used by virtualization services to interact with hardware resources.