A TRACON Controller reported an Air Carrier on Tower frequency descended below the glideslope and was below the Minimum Vectoring Altitude.

Narrative

Aircraft X had been issued 'Descend via ZZZZZ 1 arrival; cleared RNAV Y Runway XX approach.' Everything was normal during his descent on the arrival. Near ZZZZZ1 I shipped him to Tower. A few seconds later I saw him at 1;700 ft. just inside of ZZZZZ2. The MVA is 3;800 ft.; the next crossing fix is 2;000 ft.; and the Emergency Obstruction Video Map is 1;300 ft. As I noticed his altitude the low altitude alert went off. I issued the alert in the blind as Aircraft X was showing 1;400 ft. Then I called ZZZ Tower on the shout line to alert them since Aircraft X was on their frequency. I observed Aircraft X climb back up to 2;200 ft. at ZZZZZ3; where the crossing is 2;000 ft.. Aircraft X landed without incident.If the aircraft's altitude readouts were correct this was an unbelievably dangerous situation. The aircraft was 100 ft. above the ground; 210 kts. indicated at about 8 miles from the airport. I think it needs to be determined if it was malfunctioning equipment in the aircraft or a pilot error so that this doesn't happen again.