The standard answer is that 全然 is most naturally paired with negatives, and 絶対 with positives. However, modern Japanese has seen more usage of 全然 as meaning both "not at all" and also "completely, absolutely," replacing 絶対, and vice-versa. The end result is that in most cases, using 全然 or 絶対 yields the "same" meaning, but the context is such that you'd typically want to use 全然 for negatives, and for positives where the affirmation is negative, such as 全然大丈夫 "It's completely alright" (colloquially, "It's not a problem at all"), and 絶対 for stronger positive affirmations. 絶対大丈夫 comes off way stronger than 全然大丈夫, despite the fact they mean basically the same thing. It's a way of downplaying your tone.