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Windows Hypervisor
The Windows Hypervisor is a thin layer of software that allows multiple operating systems to run
simultaneously on a single physical server, and is the core component of Hyper-V. The Windows Hypervisor is
responsible for the creation and management of partitions that allow for isolated execution environments. As
shown in Figure 1, the Windows Hypervisor runs directly on top of the hardware platform, with the operating
systems running on top.
Parent and Child Partitions
To run multiple virtual machines with isolated execution environments on a physical server, Hyper-V
technology uses a logical entity called a partition. These partitions are where the operating systems and its
applications execute. Hyper-V defines two kinds of partitions, parent and child.
Parent Partition
Each Hyper-V installation consists of one parent partition, which is a virtual machine that has special or
privileged access. Some documentation might also refer to parent partitions as host partitions. This document
uses the term parent partition.
The parent partition is the only virtual machine with direct access to hardware resources. All of the other virtual
machines, which are known as child partitions, go through the parent partition for device access.
To create the parent partition, enable the Hyper-V role in Server Manager and restart the system. After the
system restarts, the Windows hypervisor is loaded first, and then the rest of the stack is converted to become
the parent partition. The virtualization stack runs in the parent partition and has direct access to the hardware
devices. The parent partition then creates the child partitions that house the guest operating systems.
Child Partition
Hyper-V executes a guest operating system and its associated applications in a virtual machine, or child
partition. Microsoft documentation sometimes also refers to child partitions as guest partitions. This document
uses the term child partition.
Child partitions do not have direct access to hardware resources, but instead have a virtual view of the
resources, which are referred to as virtual devices. Any request to the virtual devices is redirected via the
VMBus to the devices in the parent partition. The VMBus is a logical channel that enables inter-partition
communication.
The parent partition runs a Virtualization Service Provider (VSP), which connects to the VMBus and handles
device access requests from child partitions. Child partition virtual devices internally run a Virtualization Service
Client (VSC), which redirects the request to VSPs in the parent partition via the VMBus. This entire process is
transparent to the guest OS.
Integration Services
Integration services are made up of a number of services that are installed on the guest OS that improve
performance while running under Hyper-V: enlightened I/O and integration components. The version of the
guest OS deployed determines which of these two services can be installed on the guest OS.
Enlightened I/O
Enlightened I/O is a Hyper-V feature that allows virtual devices in a child partition to use host resources better
because VSC drivers in these partitions communicate directly with VSPs directly over the VMBus for storage,
networking and graphics subsystems access. Enlightened I/O is a specialized virtualization-aware
implementation of high-level communication protocols like SCSI that take advantage of VMBus directly,
bypassing any device emulation layer. This makes the communication more efficient, but requires the guest OS
to support Enlightened I/O. At the time of this writing, Windows 2008, Windows Vista and SUSE Linux are the
only operating systems that support Enlightened I/O, allowing them to run faster as guest operating systems
under Hyper-V than other operating systems that need to use slower emulated hardware.
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