With Hyper-V Server 2008, Microsoft's Hyper-V hypervisor is installed in the parent partition, and it provides just the bare essentials required for booting the system, providing hypervisor services, and exposing the management hooks necessary for System Center Virtual Machine Manager 2008. It does include drivers as well, but little else from Server 2008. It's not Server Core. It's much less than that: At boot time, you'll be prompted from a command line interface to configure some basic configuration options. But management occurs from the free Hyper-V management console (on Vista or Server 2008) or System Center Virtual Machine Manager 2008.
Microsoft says the performance characteristics of Server 2008 Hyper-V and Hyper-V Server 2008 are identical. But this version of the product has a few differences. It's limited to four physical processors and 32GB of RAM. There is no clustering support.