There is. Unfortunately the reviews part of X-cart seems more like a programmers after-thought and had to rush it in 20 minutes before getting off work.
You'll need a mod, such as one of these:
http://www.bcsengineering.com/store/customer-review-management-for-x-cart.html
http://www.bcsengineering.com/store/advanced_product-ratings-and-reviews-for-x-cart.html