An Overview of Server Virtualization: History, Types, and Core Concepts

Server virtualization technology dates back to IBM mainframe virtualization with z/VM, which allowed hundreds of virtual machines on z series hardware; later, Power platforms introduced PowerKVM and PowerVM (AIX virtualization) with support for vSCSI and NPIV, creating virtual I/O servers (VIOS).

The development of virtualization has been driven by factors such as resource efficiency, isolation, and manageability, as illustrated by the accompanying diagrams.

Partitioning enables the virtualization layer to allocate server resources among multiple VMs, allowing each guest OS to see only its assigned virtual hardware.

Isolation ensures that a failure in one VM (OS crash, application error, driver fault, etc.) does not affect other VMs on the same physical server.

Encapsulation stores the entire state of a VM—including hardware configuration, BIOS settings, memory, disk, and CPU state—in a small set of files, making it easy to copy, back up, or migrate the VM.

Virtualization can be classified by its degree of abstraction into full virtualization, para‑virtualization, and hardware‑assisted virtualization.

CPU virtualization faces challenges because the CPU operates at different privilege levels; executing privileged instructions directly in a VM can destabilize the host, so the hypervisor must intercept and manage these instructions.

In full virtualization, the VMM sits beneath the guest OS, translating privileged instructions and emulating hardware resources so the guest OS runs unmodified.

Para‑virtualization modifies the guest OS to replace privileged instructions with hypercalls that the VMM handles, reducing overhead.

Hardware‑assisted virtualization introduces new CPU instructions and modes, allowing the VMM and guest OS to run in separate privilege rings (ROOT and non‑ROOT), enabling the guest OS to execute core instructions directly on the hardware without VMM mediation.

The original article also provides a video link for further explanation of server virtualization concepts.