The reason theres no PS3 support on the PS4 is that there is no way to emulate the Cell CPU fast enough inside of the price and thermal envelope of the PS4. Furthermore theres no market for the low end Cell hardware thus there aren't any low power variants out there to have dedicated hardware for it like was the case with PS2 compatibility in early PS3s.
Now though, the PS4 should have more then enough grunt to allow emulation of PSOne and PS2 games purely in software just as the Wii does for old Nintendo consoles.
Likewise the XBone should easily be able to run original XBawks games by Microsoft just adding the proper libs to the XBone OS, both consoles are x86 based https://en.wikipedia.org/wiki/Xbox_t...specifications But for the same reasons as the PS4 cannot play 360 games.
Reviving old game libraries would be a cash cow for both companies just as it is for Steam, Desura, GOG.com etc. Of course both companies will almost assuredly require that you have to buy those games again instead of letting you dust off your discs.