In the family of the four enterprise hypervisors (consisting of VMware ESXi, Xen, KVM and Hyper-V), Hyper-V is the baby of the group, years younger than KVM, the next youngest product. Because of this, Hyper-V has had a rough time attempting to compete with more mature and featureful rivals. It needed to cover a lot of ground in a relatively short period of time.
Hyper-V today is a full fledged member of the four and is a full, enterprise ready hypervisor that will go toe to toe with any of the other three. Each has their strengths and weaknesses, some technical, some from licensing and some vendor and support related, but all have managed to carve out a place for themselves.
Hyper-V, because it is young and was presented not to virtualization engineers primarily but to Windows Administrators as the "backdoor" into the infrastructure decision making process, tends to be surrounded by myth. Three key myths that have doggedly refused to not remain attached to Hyper-V. Why Hyper-V can't shake the myths is uncertain, but this has hurt Hyper-V greatly and justified many claims that it used to only be installed by accident and never because someone had carefully evaluated it. That situation is getting better but some basics about Hyper-V that need to be understood are:
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Hyper-V, like any enterprise hypervisor, is a type 1 hypervisor (aka bare metal or native hypervisor.) Hyper-V, with no exceptions, runs right on the physical system itself. Every OS instance running on that box runs on top of Hyper-V. It is commonly confusing to look at the box itself and find what appears to be Hyper-V running as a role on top of Windows Server. Rest assured, this is only a view presented to the Windows Admin is in no way reflects reality. There is no version of Hyper-V and never has been any that runs in any mode except as on the bare metal. That it is installed as a "role" is purely a convenience means of portraying this with the expectation that most Windows Admins do not understand virtualization and want to see the installation process in a convenient, familiar way. No matter how, when or where Hyper-V is installed it is always a type 1 hypervisor. If this were not true, Hyper-V would not be considered a viable virtualization product for servers and the base OS would violate one of the most basic rules of IT infrastructure today - never run an OS on bare metal except when there is no other choice.
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Hyper-V is not Windows. They are two quite different things.
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Hyper-V is not a component of Windows. Hyper-V is a standalone product separate from Windows.
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Hyper-V is free. You can pay for other products to use with Hyper-V if you want, but Hyper-V itself is, and always has been, completely free. There are no exceptions to this, ever. It is a free product including all of its features.
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Hyper-V comes with no special licensing for Windows Server. Windows Server's licensing is applied evenly and equally regardless of the hypervisor chosen. There is no licensing benefit to Hyper-V over Xen, Vmware or KVM. None.
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As a best practice, Hyper-V should always be installed directly, not installed via Windows Server and the "role" function. This keeps the Hyper-V installation as lean as possible without accidentally creating any cumbersome licensing liabilities from the Windows Server control environment.
