what is the difference between PWHT and stress releaving?

Answer 1

PWHT is basically done to remove trapped hydrogen gases inside the weld whereas stress releiving is done to releive the stresses caused by welding or cold working

Answer 2

There is no difference.
The two terms are used interchangeably and both refer to the same process of heating and cooling of metals after fabrication, welding, cold working, etc, to relieve residual stresses.