By the looks of it, nothing, these are the exact same product just sold by two different stores. Produced by Kingston yet sold by x or y warehouse.
Hello, I asked the same question somewhere else and came to know that UV300 model is an earlier model and V300 is latest. Difference is UV300 is a bit faster but build quality is not so great causing it to be cheaper than V300 which is slower than UV300 but has average build quality. Both are nearly same.