Air carrier First Officer reported the yellow hydraulic system failed during climb on the A321.

Narrative

On climbout around FL 200 we received an ECAM of Yellow Hydraulic Low Pressure and almost immediately low level. We asked ATC to level off. We did the initial actions and then the Captain transferred controls to me and verified I had the radios. He asked me to notify ATC and ask to return to ZZZ while he ran the ECAM actions; follow-up; QRH; Non-normal landing distance; overweight landing; and non-routine landing procedures. He also kept the flight attendants; passengers and company in the loop while I flew the plane and got set up for the approach and landing back into ZZZ. We let ATC and the company know we would need to come to a stop on the runway and to be towed to the gate and requested emergency vehicles standing by. We proceeded with the approach and landing and complied with the QRH procedures. We came to a stop on the runway and had the ARTCC check us out and once clear we requested to be towed to the gate. We parked and ran the required checklists. ATC and the ARTCC crew did a great job. I was impressed the company was ready with the super tug and an open gate upon our return. The captain did a great job with decision making; keeping all parties informed; and dividing attention between managing the emergency and making sure the airplane was being flown safely. The Mechanic said when he opened the engine cowling; hydraulic fluid was everywhere. I don't know what caused the malfunction or how it could have been prevented.