Guest operating systems are the operating systems that run in the virtual environment. Depending on your version of Windows Server 2022, the number of guest operating systems depends on how many virtual machines can be loaded based on licensing. Windows Server 2022 Standard allows you to have the host system and two virtual machines. You can purchase additional licensing to allow more virtual machines, but the default is two virtual machines.

Windows Server 2022 Datacenter can have unlimited virtual machines along with the Hyper- V Host system. The maximum number of VMs that can run at once is 1,024.

Once you purchase the appropriate version of Windows Server and licensing, you can start creating virtual machines with guest operating systems.

The following guest operating systems have been successfully tested on Hyper- V and are hypervisor aware. Table 2.1 shows all of the guest server operating systems and the maximum number of virtual processors. Table 2.2 shows all of the guest client operating systems and the maximum number of virtual processors.

TABLE 2.1 Hyper-V  guest server operating systems

Guest operating system (server)	Maximum number of virtual processors
Windows Server 2022	64 for Generation 1 VMs and 240 for  Generation 2 VMs
Windows Server 2019	64 for Generation 1 VMs and 240 for  Generation 2 VMs
Windows Server 2016	64 for Generation 1 VMs and 240 for  Generation 2 VMs
Windows Server 2012 and Server 2012 R2	64
Windows Server 2008 R2 with Service Pack 1 (SP1)	64
Windows Server 2008 R2	64
Windows Server 2008 with Service Pack 2 (SP2)	8
Red Hat Enterprise Linux 5.7 and 5.8	64
Red Hat Enterprise Linux 6.0–6.3	64
SUSE Linux Enterprise Server 11 SP2	64
Open SUSE 12.1	64

Guest operating system (client)	Maximum number of virtual processors
Windows 11	32
Windows 10	32
Windows 8.1	32
CentOS 5.7 and 5.8	64
CentOS 6.0–6.3	64
Red Hat Enterprise Linux 5.7 and 5.8	64

TABLE 2.2 Hyper-V  guest client operating systems

Guest operating system (client)	Maximum number of virtual processors
Red Hat Enterprise Linux 6.0–6.3	64
SUSE Linux Enterprise Server 11 SP2	64
Open SUSE 12.1	64

The list of supported guest operating systems may always be extended.  Please check the official Microsoft Hyper- V site to obtain a current list  of supported operating systems: https://docs.microsoft.com/ en- us/windows- server/virtualization/hyper- v/hyper- v- on-  windows- server.

Hyper- V Architecture

This section will provide you with an overview of the Hyper- V architecture (see Figure 2.1). I’ll explain the differences between a hypervisor- aware and a non- hypervisor-a ware child partition.

FIGURE 2.1 Hyper- V architecture

VMBus		Emulation

Hyper-V hypervisor

Hardware

As you can see, Hyper- V is based on the microkernel architecture. As I stated earlier in the chapter, Hyper- V provides a virtualization layer called a hypervisor that runs directly on the system hardware. You can see that the hypervisor is similar to what the kernel is to  Windows. It is a software layer responsible for the interaction with the core hardware and works in conjunction with an optimized instance of Windows Server 2022 that allows running multiple operating systems on a physical server simultaneously. The Hyper- V architecture consists of the hypervisor and parent and child partitions.

The Windows Server 2022 operating system runs in the parent partition, and it delivers the WMI provider for scripting as well as the VM service.

Virtual machines each run in their own child partitions. Child partitions do not have direct access to hardware resources; instead, they have a virtual view of the resources, which are called virtual devices.

If you’re running a hypervisor- aware operating system like Windows Server 2008,

 Windows Server 2008 R2, Windows Server 2012, Windows Server 2016, Windows Server 2019, or Windows Server 2022 in your virtual machine, any request to the virtual devices is redirected via the high- speed bus to the devices in the parent partition, which will manage the requests.

By default, only Windows Server 2008 R2 and newer are hypervisor- aware operating systems. Once you install Hyper- V Integration Components on an operating system other than Windows Server 2008 R2 and newer, it will be hypervisor aware. Microsoft provides a hypervisor adapter to make Linux hypervisor aware.

Non- hypervisor- aware operating systems (for example, Windows NT 2003) use an emulator to communicate with the Windows hypervisor, which is slower than molasses in the winter.