Neither is truly Floated, they have an attached gas tube.
The SVD uses the long stroke Tokarov gas system of cup and a thin rod to keep the shift of weightfrom upsetting the rifle more during recoil, with basicly an AK type rotating bolt and carrier.
The PSL is straight "AK", gas piston being a part of the carrier.
The Soviet block used the rifles in a "Designated Marksman" role with a PSL or SVD (depending on which army your in) with one in each platoon to provide cover fire, aimed rapid fire and offeing longer range than the platoons basik AK's. They were to be effective out to 400 meters, and were not used often in the "Sniper" role we in the West have.
The ol' Mosin Nagant was used by most dedicated Soviet Block "Snipers" untill the 60's when it was retired, 'cept in Finnland where they only made the ol' Mosin more accurate.....