Psychology and physiology are at different levels of explanation or levels of analysislevels of analysis. The answer depends entirely on how you view the relationship between such levels and in particular causality(mental) causality within and between levels. As this is still heavily debated, this metaphysical consideration is a precursor for giving a more specific answer to your question. SomeSome options:
The physicalistphysicalist and reductionistreductionist answer is probably the simplest: they are the same thing, only physical stimulation is the only real one! According to this view, Psychology is an (epiphenomenalepiphenomenal) abstraction which is entirely realizedrealized by lower levels. Therefore, there can only be psychological stimulation in the presence of physical stimulation, but the latter may occur without the former being intrinsic to that. As an example, you present a snake to the subject to elicit psychological fear, but the physical stimulation (photons tranduced by the retina and the pressure transduced by the kutane receptorscutaneous receptors in the skin) is a necessity for this to happen. If you count neuronal activity as physical self-stimulation, then fearful memories could be cast in the same language.
Others try to keep the relationship between levels less well defined. For example, if you align more with emergentismemergentism, poetic naturalismpoetic naturalism, or the like, you acknowledge the underlying physical reality, but you want to keep the causal language such as "the snake made him fearful" within a given level of analysis rather than crossing levels. So the answer to your question is that there IS no difference, but you better act as if there is because you'll miss something or get confused if you don't.
A dualistdualist, of course, would hold that physical stimulation and psychological stimulation can (at list in principle) occur in its own right. Usually, they have great difficulties answering the very question you pose, and even though most people in their day-to-day lives tend to think dualistically (as intuitive psychologistsintuitive psychologists), very few defend dualism in academia nowadays.
There are other positions as well, and each of the above havehas many branches. But the above should give some search terms to get acquainted with the heavy players in the field.