They use different developing chemicals.

D-76 is a classic Metol / Hydroquinone (MQ) developer.

HC-110 is a Phenidone / Hydroquinone (PQ) developer. Results should be very similar to D-76. Beneficial if you suffer contact dermintitis from Metol and lasts longer in solution, hence can be supplied as a liquid concentrate rather than in powder form.

XTOL is a Phenidone / Ascorbic Acid (PC since Ascorbic Acid is Vitamin C) developer. A more recent and advanced forumlation.