The way I look at it, the big difference is between existentialism and absurdism lie in the problem of universalism. An existentialist is in many cases also going to be a Christian, possibly a Christian who is having a lot of doubt in their faith or struggling with the problem of evil, things like that. Existential philosophy tries to square the fact that we exist as moral beings but we seem to live in a world that lacks a universal concept of morality, so where does our morality even come from if it is not universal? To the existentialist, morality IS the underlying basic law of nature, and thus morality is itself a higher meaning, but morality is not applied universally, and this is a great conflict.
Absurdists, I feel, ultimately accept the fact that morality is NOT necessarily the basic underlying law of nature. Morality is subjective and it is personal, and it is messy and often falls short. I imagine that the absurdists have already gone through the existentialist crisis and come out on the other side with an acceptance of the seeming meaninglessness of it all, of the fact that our moral scruples are ultimately just a way to cope with existence and not some Higher Truth that we must strive towards.
So, in short:
Nihilism: Life is meaningless, and all pursuit of meaning is futile. — universalist, negative.
Existentialism: Life lacks inherent meaning, but we must create our own meaning in a world that often seems indifferent. — subjective (not necessarily universalist), can range from negative to positive.
Absurdism: Life is inherently meaningless, but we can choose to create and embrace our own meaning, even in the face of the absurd. — subjective, generally positive.