what is the difference between Hyper-V and Hyper-V Server?

In terms of licensing, what is the difference between Hyper-V and Hyper-V Server?

Hyper-V and Hyper-V Server have very similar names and look like very similar products at first glance. But these two products are very different in terms of licensing. Hyper-V is a role that can be added to an instance of the Windows Server 2008 R2 operating system (OS). You must have a functioning and licensed copy of the Windows Server 2008 R2 OS in order to use Hyper-V.
You can download Hyper-V Server for free, and it's a standalone product with no licensing requirements. Hyper-V Server is often confused with Hyper-V because Hyper-V Server arrives as a feature-restricted instance of Windows Server 2008 R2 Server Core Edition. That feature-restricted instance is intended to run Hyper-V services only (including add-on services such as host clustering).

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