<p>I would like to ask if it is possible to show Generalization relationship in UML for a Domain Model although I understand I can do that in a Class Diagram? I did not see much examples of Domain Model displaying Generalization Relationship except in Class Diagram. If not possible, what is the best way to display that an Entity in a Domain Model is either a Entity A or Entity B or Entity C or Entity D etc ?</p>

You need to clarify why you're distinguishing between a domain model and a class diagram.
On the one hand, your domain model could simply be a class diagram of everything that could map to your database, and consume it. Hence a generalization relationship could simply depict implementation inheritance or interface inheritance.
On the other hand, your domain model could simply express how you expect your classes to work. In which case, it could be any of the standard UML diagrams: class, sequence, collaboration, component, activity, etc.
ADD: are you talking about finding different ways to categorize your persistent entities like a tagging system? Or you could make it possible to have a persistent entity have many nodes in a category tree?