If someone has recently watched a movie, we'd expect them to say "I loved it".
The explanation is the typical one for choosing between simple past and simple present tenses in English: past describes a completed action, present describes habitual actions or feelings.
If I watched a movie last night, I'd most likely say something like, "I watched Solaris last night - I loved it."
If I watch a movie every year, I might say something like, "I love Hannah and her Sisters - I watch it every Thanksgiving."
The expectation with the present tense is that it probably takes some time, or perhaps some number of viewings, for a movie to become habitually loved. What amount of time, or number of viewings? That's not so clear. But, again, that's the same basic problem with the present tense.
How many times do you need to have eaten pizza on a Friday before you can say, "I always eat pizza on Friday"?
However, there is at least one circumstance where having just viewed a movie for the first time, you can say, "I love it". Imagine a friend sits you down in front of the computer to show you a video she's just made, or even more likely, one she's still working on. She shows you what she's done and then asks, "So, what do you think?" You could then respond, "I love it". But it also wouldn't be inappropriate to say, "I loved it".
Here, the present tense gives more of a sense of I love it so far (because it's still a work in progress), whereas the past tense suggests more of I loved it, and that's my final opinion.