What is the difference between self pollination and cross pollination? Which of these two modes of pollination contributes more to the plant diversity?

Self pollination happens when pollen grains from a flowering plant fall into the pistils of the similar plant and thus gametes from the same individual unite to form a zygote. Cross pollination occurs when pollinators carry pollen grains from a plant to reach other individual plants of the same species thus gametes of dissimilar individuals form the zygote.

As it promotes formation of zygotes containing genes from dissimilar individuals (new gene combinations) cross pollination contributes more to biological diversity.