VMM

VMM stands for Virtual Machine Monitor. It is software, firmware, or a combination that creates and runs virtual machines by abstracting the underlying physical hardware. The VMM provides an execution environment for guest operating systems and applications, while enforcing isolation between VMs and allocating hardware resources such as CPU, memory, and I/O.

VMMs are commonly categorized as Type 1 (bare-metal) hypervisors, which run directly on host hardware, and Type 2 (hosted) hypervisors, which run on a host operating system. Notable examples include VMware ESXi and Microsoft Hyper-V (Type 1), and Oracle VirtualBox or VMware Workstation (Type 2). Open source options include KVM, Xen, and QEMU. The VMM may expose virtual devices to guests through emulation or paravirtualized interfaces and leverages hardware-assisted virtualization features such as Intel VT-x or AMD-V to improve efficiency.

Key functions include CPU scheduling, memory virtualization (including techniques like paging, shadow paging, or extended page tables), I/O virtualization, and device emulation. Advanced VMMs support live migration of running VMs, snapshots, and cloning, enabling dynamic resource management and high availability in data centers and cloud environments. Security and isolation rely on strong separation of VM namespaces and careful access control.

Historically, the term VMM is sometimes used interchangeably with "hypervisor" and sits at a different layer than container-based virtualization, which shares the host OS kernel. VMM-based virtualization remains central to cloud infrastructure, enterprise data centers, and virtualization research.