Diesel-electric. They cruise around just under the surface on diesel, with snorkels to bring in air and expel exhaust. But, then they can shut down those diesel engines, fully submerge, and maneuver on batteries for a few days and maybe a hundred miles.

While they are on diesel engines, they are loud, and stuck to the surface. While they are on batteries, they are silent. For the few days that they are submerged, they are quieter than our nuclear subs.

Yeah, they pose a potential threat to a carrier group, but the "proportional response" to attacking a carrier would be the destruction of every naval facility they have, so not exactly a serious threat.

I don't know anything about Iran's subs, but Sweden's famously stealthy diesel subs keep a big tank of liquid oxygen on board and mix the exhaust with seawater before releasing it

Diesel subs use snorkles when using their engines and batteries for silent running. They work a lot like diesel/electric trains, in that the diesel engine is acting more like a generator for an electrical engine.

IIRC from my C:MO experience, diesel subs (at least Greek ones that is) only use diesel above surface, and use batteries when submerged

The short answer is “it’s complicated but yes, and practically no.” Nuclear submarines have the operation range to obfuscate their location orders of magnitude better than diesel. Diesel is quieter but their range makes tracking infinitely more feasible. “A needle in a haystack vs a splinter in pail of hay.” Nuclear is better at power projection, diesel is better at short range defense/offense.

Look at it this way: When a diesel sub is endangered, they can turn off everything and I mean EVERYTHING. Quiet as a mouse that doesn't exist. However, it will have to resurface eventually. A nuclear sub cannot do that. The cooling pumps have to keep running. But they can stay under water pretty much indefinitely (until they run out of food).

Nuclear is quieter than diesel, but louder than electric. Diesel subs are on diesel for transit to/from their patrol area, and on battery for their short-range patrol. Nuclear subs are much quieter during transit, but slightly louder during a long-range patrol.

Nuclear subs have to continuously pump cooling water; diesel-electrics can shut down pretty much everything that makes noise.

Nuclear subs have cooling systems that need to be working constantly. This creates additional vibrations/noise on top of the propulsion system.s that would be pretty similar between both subs. As well as nuclear subs needing to be much bigger, so require propulsion that produce more noise

Diesel submarines can literally turn off every system and just drift in the ocean currents silently, or loiter on the sea floor where they are indistinguishable from a rock.

Worth noting nuclear submarines have a sort of minimum-practical-size determined by the need for a functional nuclear reactor on board. Combined with the plain expense of nuclear reactors means that states can build way more ssk's than ssn's for a given budget. It's often better to have three 25% chances of sinking the other guy than one 50% chance.