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SQL Server 2008 does perform very well in a Hyper-V environment when properly configured, which among other things means don't use the default of dynamically expanding virtual hard disk. Hyper-V isn't at all like Virtual Server or Virtual PC. It's best to drop preconceptions drawn from experience with those products.
For best I/O performance in Hyper-V, use passthrough disks (this is when a physical disk is assigned directly to Hyper-V without using a vhd file). Fixed size virtual hard disks are a close second. Dynamically expanding disks and differencing disks provide convenience at the cost of performance and thus should not be used when I/O performance must be maximized.
High network traffic imposes a burden on the CPU when running on Hyper-V (this is also true of Virtual Server). If a SQL Server configuration is CPU bound on bare metal, it might not have enough CPU cycles left for optimal networking performance. Also, keep in mind that a maximum of four logical processors can be assigned to a Hyper-V virtual machine, so if your SQL Server configuration needs more than four processors, it is not suitable for Hyper-V.