Hardware/platform/server virtualization

During the pre-virtualization era, a physical machine was considered a singleton entity that could host one operation system and could contain more than one application. Enterprises that run highly critical businesses or multitenant environments need isolation between applications. This limits from using one server for many applications. Hardware virtualization or VM virtualization helped to scale out single physical servers as they host multiple VMs within a single server where each VM can run in complete isolation. Each VM's CPU and memory needs can be configured as per the application's demand.

A discrete software unit called hypervisor or Virtual Machine Manager (VMM) runs on top of virtualized hardware and facilitates server virtualization. Modern cloud platforms, both public and private, are the best examples of hardware virtualization. Each physical server runs an operation system called host OS, which runs multiple VMs each with their own OS called guest OS. The underlying memory and CPU of the host OS is shared across the VMs depending on how the VMs are configured while creating. Server virtualization also enables hybrid computing, which means the guest OS can be of any type, for example, a machine running Windows with Hyper-V role enabled can host VMs running Linux and Windows OSes (for example Windows 10 and Windows 8.1) or even another Windows Server OS. Some examples of server virtualization are VMware, Citrix XenServer, and MS Hyper-V.

In a nutshell, this is what platform virtualization looks like: