What's difference between " I knew i loved you" and " i knew i love you"?

The first one reads more naturally because it refers to a moment in time where she first became sure of his love. It assumes that his love is a continuous event from then until now.

If you say "I knew" you are saying that you had that knowledge in the past but not that you have that knowledge now. You might have it now but the sentence does not say so.

If you say "I know" you are saying that you have that knowledge now.

If you say "You loved me" you are saying that the other person loved you in the past but not that the other person still loves you. Again they might do but the sentence does not say so.

If you say "You love me" you are saying that the other person loves you now.

If you say "I knew you loved me" you are saying that, at some point in the past, you had the knowledge that the other person loved you at some time in the past. The knowledge and the love might still both exist or they might not, they might have been concurrent or the love might have started or even have started and ceased before you knew of it. The sentences do not say. However it is not possible that the knowledge preceded the love because that would be a prediction and require you to say "I knew that you would love me.

You could not say "I knew in January 2014 that you loved me in August 2015" because when you had the knowledge the fact of the love was in the future. You could say "I knew in January 2014 that you would love me in August 2015". We might ask how you knew it but the sentence makes sense because you were predicting in 2014 what was then a future event even though both the prediction and the event are now both in the past

The same condition exists with your second sentence. If you say "I knew that you love me" this makes no sense because when you had the knowledge the love was in the future. You could say "I knew that you would love me" because you were then predicting what was then a future event.

All of the following make sense:

1 "I knew you loved me" (both knowledge and love were in the past)

2 "I knew that you would love me" (the knowledge is in the past but the love could be any time after that, it might be in the more recent past, the present or even the future"

3 "I know you loved me" (the knowledge is in the present but the love is in the past)

4 "I know that you loved me then and that you still do" (the knowledge is in the present but the love was in the past and continues to exist)

5"I know that you love me" (both love and knowledge are in the present)

You can even say

6 "I know that you will love me" (the knowledge is in the present but the love is in the future)

However "I knew that you love me" is grammatically incorrect since the knowledge is in the past and the love is in the present, which was the future when the knowledge existed. You can only use the form of sentence 2 in those circumstances.