Question for someone running Server 2012 datacenter edition without VMM... Can you put 2012 R2 guests on that guy or do you have to upgrade 2012 to R2 first?
I think generation 2 was implemented first in 2012R2 so you only have the option to use generation 1.
I would suspect you cannot run gen2 guests on 2012, as 2012R2 introduced that.
2012 and lower only supports Gen 1, and 2012 R2 supports Gen 1 and Gen 2.
There is no automatic way to convert a VM from Gen 1 to Gen 2, however I saw a script on Aidan Finn's site that can do it. I haven't messed with it myself yet. It also only works one way, no way to go back to Gen 1.