"I love you" describes your state of mind at the moment. "I have loved you" implies that I started to love you in the past and I still love you or it could mean I don't love you now but you have to look at at the wider context. The third structure is not normally used, since state verbs don't make much sense in the progressive form.
The progressive aspect(not tense) generally implies incompletion, temporariness, and continuity. However, you have to consider the meaning of the verb.
Student or Learner