Virtual Machine (VM) is a virtualized environment created by the virtual machine monitor that emulates a server and is capable of running an operating system. In fact, unmodified operating systems and executables are capable of running within a virtual machine. Multiple virtual machines are capable of running within a single VM host, and each virtual machine retains a high level of software isolation. Since virtual machines rely on software to provide operating system isolation and resource sharing, they are often referred to as soft partitions.
VM Guest is an operating system running within a virtual machine. The terms "virtual machine" and "VM guest" are often used interchangeably because a VM guest often refers to the guest operating system and the virtual machine as a whole. Every VM guest is allocated a portion of the physical system's CPU, memory, and I/O resources. Multiple VM guests can share CPU and I/O resources; however, the VM guests are unaware the resources are shared. The sharing is handled in the VM host by the VM application.
VM Host is an operating system that acts as the controller for the VM guests. The VM host owns the hardware and performs all of the hardware-dependant operations on behalf of the virtual machine monitor. The VM host is the operating system where management of the physical system is performed and where the VM guests are configured and managed.
Virtual CPU is a CPU allocated to a VM guest. A virtual CPU can be backed by the equivalent of an entire physical CPU or a portion of a CPU in the VM host. In any case, the VM guest is unaware of the mapping between virtual CPUs and physical CPUs. From the VM guest's perspective, it has visibility and complete ownership of the number of CPUs assigned to the VM guest. The VM host is responsible for ensuring that each VM guest receives its minimum CPU entitlement.
Virtual CPU Entitlement is the minimum allocation of CPU resources a VM guest is entitled to. If a VM guest is not consuming all of its entitlement, other VM guests are permitted to utilize the unused resources. When a VM guest requires more resources, it is allowed to borrow from other VM guests provided the resources are not being used. Virtual CPU entitlements are discussed further in the following section.
Virtual Network Switch (vSwitch) is a software component that runs on the VM host. Virtual network switches allow multiple VM guests to connect to one another for a high-speed internal network within the VM host. In addition, virtual network switches allow multiple VMs to connect to the physical network and share network interface cards. Other nodes on the physical network are unaware a virtual network switch is being used when communicating with a VM because every virtual interface has a unique hardware address that is used on the physical network. A virtual network switch is also referred to as a vswitch.
VM Application (vm_app) runs on the VM host and allocates resources on behalf of the VM guest. There is one instance of the vm_app running on the VM host for every running VM guest. The vm_app appears as a process on the VM host and contains one kernel thread for every virtual processor allocated to the VM guest. The vm_app's memory usage is the same as the amount of memory assigned to the VM guest. Finally, the vm_app has open devices for all of the virtual devices assigned to the VM guest.
VM Driver (VMDVR) is a dynamically loadable kernel pseudo-driver that is loaded on the VM host. The purpose of the VMDVR is to perform tasks such as creating, starting, and stopping VM guests at the request of system administrators.
VM Monitor (VMM) is responsible for providing virtualized Itanium processor family (IPF) processors and a virtualized IPF platform.
