How can gravitions exist without violating GR, since GR says that gravity is curvature in space-time.
For energies below the scale where gravity becomes strongly coupled, the paradigm of QFT is applicable and teaches us that gravity is due to the exchange of massless spin-2 particles we call gravitons, and that the whole picture of curved space time is nothing but a nice way to equivalently represent the collective effect of a huge number of gravitons (classical limit of a quantum system), so that it "seems" as if space time is curved for large objects. Of course this whole picture can be wrong, an there might never be gravitons, but it is in this pictures that the notion of a graviton has meaning.
GR predicts gravitational waves, which hence exist without violating it. By the wave-particle duality, there must be a particle (or at least a quasi-particle) associated with (the quantization of) such waves. We arbitrarily name them gravitons. Whatever aspect of them may be found to violate GR simply indicates how we need to amend our theory on the way to a consistent theory of quantum gravity.