You can see here that it is (-1).
http://pdglive.lbl.gov/Rsummary.brl?nodein=S011&sub=Yr&return=MXXX020
Note that for mesons, parity only depends on the orbital angular momentum L as
P=(-1)^(L+1)
but since J=0 for K0, and the parity is negative, that implies that L=0 and s=0 are zero too, therefore they will be zero too for the K0 bar, and the parity will be also (-1).
Since E = mc^2, how can photons be massless? If a photon has no mass, then, according to Einstein's formula, its energy is given by E = 0 x c^2, which is 0. Yet, photons do have energy. This seems to be a complete contradiction. Please explain! Thank you.