There is no difference, jiena is the long version and jien the short version. I don't really know why there are two versions. I only know that they are both used and both accepted in spoken and written language. Personal pronouns in the singular have all a long and a short version. Jien or jiena Int or inti Hu or huwa Hi or hija Personal pronouns in the plural are always the same Aħna Intom Huma Inselli għalik

Jien is masculine and jiena is feminine I believe