in cystalization,we heat the solution to get a saturated solution, but it is said that solubility increases on heating a saturated solution and it even becomes an unsaturated solution

During crysatllization the solution contains more solute molecules  or ions than the solute molecules present during saturation (i.e during equilibrium .Thus when heated  during crystallisation the crysatl growth ocurs due to formation of clusters  where as in saturation when heated the clusters  are not formed as the solute molecules are not much.