To understand such a difference, I recomend you use Google image for both words. :) Now, I will help you with this one :
"Une cuve" is more specific than "Un réservoir", I mean that "Un réservoir" is more often used because it refers to more objects and it is, to me, more natural in many situation.
"Une cuve" is also very natural but for some reason I hear it less often even though it means the same.
I am not telling you to use "réservoir" instead of "cuve" but I want to make you undrerstand that you will hear "réservoir" more often, I guess.