Weightage of a review paper on an undergraduate CV
Yes, it holds a weight.
How much? it depends on the sensibility of the Panel, if no hard-rules are set.
Maybe 0.5 times the weight of a regular paper, because it is missing the novelty? Or maybe 1.5 times the weight of a regular paper, because if you took part in the review, then you already are updated about the State-of-Art on the topic and it can be very relevant to your PhD?
In absence of hard rules, it depends strongly on how you frame it. Be smart.
Then of course it has not much weight in the CV, if it is a 10 years old review paper: in 10 years the SoA changed a lot ... and if did not change much, you can publish an update of the same review paper, updating the content.