The difference would be (very simplified) the one between poking a controlled hole with a needle in an only somewhat inflated water balloon now, causing a small but steady stream of water to push out, vs. waiting until it fills up and bursts completely, one is way more violent than the other.
Yes, they are. You're probably thinking of pyroclastic flow, which is also quite violent on its own. It's the waterfall of everything in the eruption column falling back to earth, and contains solid and molten rock, ash, deadly chemicals, and superheated gas, all traveling at hundreds of miles per hour.
>>A supervolcano is a volcano that has had an eruption with a Volcanic Explosivity Index (VEI) of 8, the largest recorded value on the index. This means the volume of deposits for such an eruption is greater than 1,000 cubic kilometers (240 cubic miles).
The most recent supervolcano eruption was what formed Lake Taupo, New Zealand:
>>The Oruanui eruption of the Taupō Volcano was the world's largest known eruption in the past 70,000 years, with a Volcanic Explosivity Index of 8. It occurred around 26,500 years ago and generated approximately 430 km3 (100 cu mi) of pyroclastic fall deposits, 320 km3 (77 cu mi) of pyroclastic density current (PDC) deposits (mostly ignimbrite) and 420 km3 (100 cu mi) of primary intracaldera material, equivalent to 530 km3 (130 cu mi) of magma.
>>Modern Lake Taupō partly fills the caldera generated during this eruption. Tephra from the eruption covered much of the central North Island with ignimbrite up to 200 m (660 ft) deep.
I do not get your point, as where do those ash and gases from if not said super volcano erupting due to high pressure - which can mean both higher pressure than previously or less pressure resistance of the chamber, or a combination of that, which could be reduced by letting out a steady magma flow earlier which itself doesn't mean there has to be lots of ash or gases getting spewed out (i.e., the violent part).
