Which is better for the environment, superglue or pva glue?

PVA is biodegradable. Clear glue in a tube that has a strong solvent smell (ie. Superglue) is toxic to aquatic organisms and may cause long-term adverse effects in the aquatic environment.

How do they each work?

Superglue and PVA glue both use polymerisation for adhesion. Polymerisation is a chemical reaction, where the sharing of electrons allows some types of molecule to join together, forming long chains called polymers. Superglue contains a compound called ethyl 2-cyanoacrylate, which reacts with water to cure (ie turn from a liquid into a solid). Usually there is a very thin layer of water present on most surfaces – condensed water vapour from the air – making superglue effective on almost any type of surface. PVA (polyvinyl acetate) glue is water-based, and is commonly used for sticking wood together. Water in PVA intertwines the fibres of wood with the polyvinyl acetate, eventually evaporating and leaving behind a bond stronger than the material itself.