http://msdn.microsoft.com/en-us/windows/hardware/gg463149.aspx
The main benefits to using a generation 2 virtual machine are the ability to boot from
the synthetic SCSI controller or network device, and the minimized risks associated
with boot-time malware (by leveraging the UEFI Secure Boot capability). In Windows
Server 2016, the new virtual TPM and shielded VM functionality are available for only
generation 2 VMs. Installing and booting an operating system take less time on a
generation 2 virtual machine compared to a generation 1 virtual machine, but after the
virtual machine has booted, there is no performance difference. The choice of
generation 1 vs. generation 2 is made when the virtual machine is created and cannot
be changed. A single Hyper-V server can have a mix of generation 1 and generation 2
virtual machines.
When deciding to use generation 1 or generation 2, my advice is to use generation 2, if
you do not need backward compatibility with Windows Server 2012 Hyper-V.
Compatibility is not required with other public cloud services either, such as Windows
Azure Infrastructure as a Service (IaaS), which at the time of this writing does not
support generation 2 virtual machines; this will change over time.
CONVERTING A GENERATION 1 VIRTUAL MACHINE TO
GENERATION 2
The question of converting a generation 1 virtual machine to a generation 2
virtual machine comes up often, but the reality is that you don’t need to do this in
most cases. Generation 1 virtual machines will continue to work and perform the
same as a generation 2 virtual machine. But what if you really want to make this
conversion? You can’t, or at least not without a huge amount of work.
A generation 1 virtual machine is BIOS based, which equates to a certain disk
configuration such as an NTFS system partition. A generation 2 virtual machine is
UEFI based and uses a FAT32 system partition. This alone prohibits moving
virtual hard disks between generation 1 and generation 2 virtual machines. Also
remember that generation 1 machines boot from the IDE controller, and
generation 2 machines boot from the SCSI controller.
The only way to move from generation 1 to generation 2 is to boot the virtual
machine from Windows PE, capture the partitions to a WIM file, and then
redeploy to a generation 2 virtual machine. This amount of effort, however, is not
worth the benefit. Generation 2 is best saved for new virtual machines.VMCX Configuration File
Prior to Windows Server 2016, the configuration for a virtual machine was stored in
an XML file that was readable in an XML or text editor. The file could also be edited to
make changes to the configuration outside supported mechanisms, such as Hyper-V
Manager and the Hyper-V PowerShell cmdlets, which could lead to problems.