I have a question that is bothering me since few weeks. I'm not a chemist, nor I have any experience with polymers theory, but:

Epoxy - is defined on Wikipedia as a thermoset polymer as a process of polimerization occurs that can not be reversed.

PVA - Is defined (again on Wikipedia) as a thermoplastic polymer.

So why the cured epoxy has a glass transition temperature, and therefore when heated above that temperature it softens and its shape can be modified. But when PVA glue hardens (by loss of humidity) it can not be softened by heat?

Epoxy has a glass transition due to a loss of crystallinity but is not usually shapeable unless formulated to or poorly prepared. PVA is a thermoplastic that melts around $\pu{200^\circ C}$ which is also the temperature that it starts to pyrolyze and char. With careful temperature control you can melt and form PVA and there are 3D filaments dissolvable in water based on PVA. Without careful control, you decompose the plastic before melting it.