The velocity of a particle depends on the chosen reference frame to describe the motion. If you choose some inertial reference frame, you can always choose another one moving at constant velocity to the first, which will be an equally valid inertial reference frame. The kinetic energy of a particle in this new frame will be different, but the physics will be the same. So just as there is no absolute potential energy (because the potential energy value will depend on the zero you have chosen), there is no absolute kinetic energy-- kinetic energy will depend on the chosen frame. It's the relative differences of these quantities that matter.