ag.algebraic geometry - Projective Curves which are Principal Bundles

CP^2 is not a curve. So you may have misstated your question. Nonetheless, here is my answer:

Every curve of genus 1 is a principal homogenous space for its Jacobian. Over an algebraically closed field, a principal homogenous space is just the group itself, and that is what happens in this case.

For genus g >= 2, no algebraic curve has more than 84(g-1) algebraic automorphisms. In particular, no curve can be a homogenous space.

EDIT The comment about CP^2 refers to an earlier version of the question.