virtualmachine

A virtual machine (VM) is a software-defined computer that runs an operating system and applications as if it were on physical hardware, but on top of a host system managed by a software layer called a hypervisor. The hypervisor abstracts and allocates physical resources such as central processing units, memory, storage, and networking to one or more VMs, enabling multiple isolated environments to share the same physical machine.

Two main categories of hypervisors exist: Type 1 (bare-metal) runs directly on the host hardware; Type 2 (hosted) runs within a conventional operating system. In full virtualization, the guest OS does not need modification; in para-virtualization, the guest cooperates with the hypervisor to reduce overhead. Modern CPUs provide virtualization extensions (Intel VT-x, AMD-V) that help improve performance by allowing the VM to execute privileged instructions without repeatedly leaving to the hypervisor.

Virtual machines present virtualized hardware: virtual CPU, memory, storage (virtual disks), and virtual network interfaces. They can be created, started, paused, migrated between hosts, snapped, cloned, or templated for rapid provisioning. Live migration moves a running VM with minimal downtime, facilitating maintenance and balancing loads.

Common uses include server consolidation, testing and development, disaster recovery, and cloud computing services (infrastructure as a service). Popular platforms include VMware vSphere, Microsoft Hyper-V, KVM, and Xen. Security considerations include strong isolation between VMs, careful patching, and defense against VM escape or side-channel attacks.

History notes: virtualization began on mainframes in the 1960s–70s, with IBM's VM/370 being influential. Modern VMs are central to IaaS and enterprise data centers.