VMMs

VMM, short for virtual machine monitor, is software, firmware, or a combination that creates and runs virtual machines (VMs) by abstracting and partitioning the physical hardware. A VMM allocates CPU time, memory, storage, and I/O resources to each VM while enforcing isolation between them, allowing multiple operating systems to run on a single physical host.

VMMs are commonly categorized by their placement relative to hardware. Type 1, or bare-metal hypervisors, execute directly on the host's hardware and manage guest VMs without a host OS. Type 2, or hosted hypervisors, run as software applications atop a conventional operating system. Examples of Type 1 VMMs include VMware ESXi, Microsoft Hyper-V, and the Linux-based KVM when used in a bare-metal configuration; Type 2 VMMs include Oracle VirtualBox, VMware Workstation, and Parallels Desktop.

Modern VMMs employ hardware-assisted virtualization, using processor extensions such as Intel VT-x or AMD-V to trap and virtualize privileged instructions. Paravirtualization can be used to reduce overhead by having guest operating systems cooperate with the VMM. Memory management techniques such as second-level address translation (SLAT) or extended page tables (EPT) and I/O virtualization methods (for example virtio or SR-IOV) improve performance and efficiency.

Typical use cases include server consolidation, private cloud deployments, software development and testing, and secure sandboxing of untrusted workloads. Security considerations focus on enforcing strong isolation, minimizing trusted code, and protecting against vulnerabilities that could enable VMM escapes or side-channel attacks.

See also: hypervisor, virtualization, cloud computing, containerization.