hypervisor

A hypervisor, also known as a virtual machine monitor (VMM), is a software, firmware, or hardware layer that enables multiple virtual machines to run on a single physical host by abstracting and multiplexing the underlying hardware resources. Each VM runs its own guest operating system and applications, while the hypervisor enforces isolation and controls access to CPU, memory, storage, and network I/O.

Hypervisors are commonly categorized as Type 1 (bare-metal) and Type 2 (hosted). Type 1 hypervisors execute directly on host hardware, providing high performance and security; typical examples include VMware ESXi, Microsoft Hyper-V, and Citrix Hypervisor. Type 2 hypervisors run on top of a general-purpose operating system, mainly for desktop or test environments; examples include Oracle VirtualBox, VMware Workstation, and Parallels Desktop.

Modern hypervisors employ various virtualization techniques. Hardware-assisted virtualization uses CPU extensions such as Intel VT-x or AMD-V to achieve full virtualization with minimal performance penalties. Some environments employ paravirtualization, where the guest OS cooperates with the hypervisor to optimize operations. I/O virtualization, memory ballooning, and other mechanisms help improve efficiency and flexibility in resource management.

Key responsibilities of a hypervisor include VM lifecycle management, resource scheduling, memory overcommit and ballooning, device emulation and I/O virtualization, live migration of running VMs, snapshots, and ensuring isolation between VMs. Hypervisors underpin many cloud and data center architectures, enabling server consolidation, scalable testing, disaster recovery, and multi-tenant cloud services.

The hypervisor ecosystem encompasses both proprietary products and open-source projects, reflecting a shift toward pervasive virtualization in enterprise IT and cloud computing.