  Using realistic quark propagators and meson Bethe-Salpeter amplitudes based
on the Dyson-Schwinger equations, we calculate the pion and kaon's valence
parton distribution functions (PDF) through the modified impulse approximation.
The PDFs we obtained at hadronic scale have the purely valence characteristic
and exhibit both dynamical chiral symmetry breaking and SU(3) flavor symmetry
breaking effects. A new calculation technique is introduced to determine the
valence PDFs with precision. Through NLO DGLAP evolution, our result is
compared with pion and kaon valence PDF data at experimental scale. Good
agreement is found in the case of pion, while deviation emerges for kaon. We
point out this situation can be resolved by incorporating gluon contributions
into the mesons if the pion hosts more gluons than kaon nonperturbatively.
