I saw - Simple Past Tense (The Past Tense describes the things you did yesterday, last year, back in February, in 1973, or an hour ago.)
I have seen= Present Perfect Tense ( We use this tense for an action that began in the past and continuous up to now. )The formula for this tense is :
Subject + Has or Have + the Past Participle
I have seen this movie many times.
Hope this helps.
