A service model is the way that a firm offers intangible value to its customers. Different XaaS concepts describe broad category of service models, which offer customers product delivery and payment options that allow them to purchase access to products as a service. the most common ones are the three general cloud computing models: Software as a Service (SaaS), Platform as a Service (PaaS) and Infrastructure as a Service (IaaS). See also https://en.wikipedia.org/wiki/Cloud_computing#Service_models.
Infrastructure as a Service (IaaS) are online services that provide high-level APIs used to dereference various low-level details of underlying network infrastructure like physical computing resources, location, data partitioning, scaling, security, backup etc. (From https://en.wikipedia.org/wiki/Infrastructure_as_a_service)
Software as a service (SaaS /sæs/) is a software licensing and delivery model in which software is licensed on a subscription basis and is centrally hosted. It is sometimes referred to as "on-demand software". (from https://en.wikipedia.org/wiki/Software_as_a_service)
Platform as a Service (PaaS) or application platform as a Service (aPaaS) or platform base service is a category of cloud computing services that provides a platform allowing customers to develop, run, and manage applications without the complexity of building and maintaining the infrastructure typically associated with developing and launching an app. (from https://en.wikipedia.org/wiki/Platform_as_a_service)
Product as a service covers business models where the manufacturer or brand owner retains the ownership of an asset and offers it to customers as a service. In this business model, the company offering the product has an incentive to optimize the use and life cycle of the asset.