From what I would assume, the two major forces acting on the string are gravity and the wind. There is some percentage of the force of the wind that is giving the kite lift, but assuming the kite's position is fixed, you have a force in one direction that forms the catenary curve, and then another in a perpendicular (to simplify things) direction that is dependent on the angle of the string with relation to the wind vector, assuming a simple wind system with a constant vector.
The wikipedia page for catenary curves goes into the shape of catenary curves under a horizontal force:
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