Hello Ajay
When we talk about services running on systems of any kind (servers, cloud, or whatever else…) these systems are broken down into the following components:
The varying cloud service models essentially define which of these components are delivered by the platform.
Middleware, according to the Google dictionary is defined as:
software that acts as a bridge between an operating system or database and applications, especially on a network.
Wikipedia includes a detailed description of an example of middleware:
The Android operating system uses the Linux kernel at its core, and also provides an application framework that developers incorporate into their applications. In addition, Android provides a middleware layer including libraries that provide services such as data storage, screen display, multimedia, and web browsing. Because the middleware libraries are compiled to machine language, services execute quickly. Middleware libraries also implement device-specific functions, so applications and the application framework need not concern themselves with variations between various Android devices. Android’s middleware layer also contains the ART virtual machine and its core Java application libraries.
Runtime is a term used to refer to the period of time during which a computer program is executing. In this context, cloud platforms that are responsible for runtime are responsible for providing the compute resources and memory to run the application. In other words, the application in question is running on CPU and memory provided by the cloud services provider.
I hope this has been helpful!
Laz