In a basic sense, the difference between SaaS marketing and product marketing is that SaaS marketing is about generating demand for a SaaS product, while product marketing is about creating and communicating the value of that product.
SaaS marketing focuses on all the marketing activities involved in building brand awareness, generating leads, and turning those leads into long-term customers. Product marketing, on the other hand, is all about ensuring potential customers understand how the software (the product) and its features can benefit them.