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