  We study the chiral behavior of the electromagnetic (EM) form factors of pion
and kaon in three-flavor lattice QCD. In order to make a direct comparison of
the lattice data with chiral perturbation theory (ChPT), we employ the overlap
quark action that has exact chiral symmetry. Gauge ensembles are generated at a
lattice spacing of 0.11 fm with four pion masses ranging between M_pi \simeq
290 MeV and 540 MeV and with a strange quark mass m_s close to its physical
value. We utilize the all-to-all quark propagator technique to calculate the EM
form factors with high precision. Their dependence on m_s and on the momentum
transfer is studied by using the reweighting technique and the twisted boundary
conditions for the quark fields, respectively. A detailed comparison with SU(2)
and SU(3) ChPT reveals that the next-to-next-to-leading order terms in the
chiral expansion are important to describe the chiral behavior of the form
factors in the pion mass range studied in this work. We estimate the relevant
low-energy constants and the charge radii, and find reasonable agreement with
phenomenological and experimental results.
