Mixin classes provide a specialized set of functionality. Eg: rendering a template, JSON encoding, fetching an object from the database. They can not be used without the View class.
BaseView classes (eg: BaseDetailView) provide a full set of logic, except the rendering of the response. This makes them highly reusable. For example combined with the JSONResponseMixin, they can render JSON responses, combined with the TemplateResponseMixin, they can render template responses.
View classes (eg: TemplateView) provide a full set of logic, including the rendering of responses.