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Generation 1 Provides the same virtual hardware to the virtual machine as in previous versions of Hyper-V.
Generation 2 Provides the following new functionality on a virtual machine:
PXE boot by using a standard network adapter
Boot from a SCSI virtual hard disk
Boot from a SCSI virtual DVD
Secure Boot (enabled by default)
UEFI firmware support
IDE drives and legacy network adapter support has been removed
Informações adicionais sobre cada marcador estão disponíveis no artigo do technet vinculado acima.