' I love you ' - I am in love with you now ' I loved you ' - I used to love you / I loved you before [but I do not love you now]

It all depends in the context: "I loved you" (by itself): suggests that I used to love you, but I do not love you any more But you can say, for example: "I loved you the first time we met". In that context, there is no suggestion that my love for you has stopped. I could still be loving you (or not)