One difference between a fundamental niche and a realized niche is that

Ecological niche refers to the functional role an organism perform in an ecosystem. It describes how an organism meet its need for food and shelter, how it reproduce, how it interact with the other species in the ecosystem and the physical environment.

The fundamental niche is the the range of ideal environmental conditions in which an organism or species can survive or the set of conditions which are suitable for the survival of the organism in an ecosystem. The realized niche is the range of environmental conditions, which an organism or species actually use for its survival.  

The main difference between a fundamental niche and a realized niche is the chances of survival of a species in a fundamental niche is expected to be higher than in a realized niche. As, the conditions in a fundamental niche are ideal and more favorable for the survival of the species or organism than realized niche.