In which meetings the Key Stakeholders are allowed to participate?

Allowed is a pretty strong word in my opinion.  I think the better question is "In which events would the Key Stakeholder's presence be most useful".   But you have realize the difference between meetings and events.  The Scrum framework describes 5 events but no meetings.  Events usually have a specific purpose where a meeting can be used to address many different things. 

The Scrum Guide uses the word "stakeholder(s)" 13 times.  Most of them are in reference to how stakeholders provide valuable information and that meeting stakeholder's desires are important. The only event (not meetings) where the key stakeholders are referenced is the Sprint Review.  However, there is one other of the events that might include them if their presence is considered warranted.  Read this statement where the Sprint Planning event is described

The Product Owner ensures that attendees are prepared to discuss the most important Product Backlog items and how they map to the Product Goal. The Scrum Team may also invite other people to attend Sprint Planning to provide advice.

To me that means that stakeholders could be invited if the Scrum Team feels their presence is beneficial. 

As for the other events.  

The Daily Scrum is a 15-minute event for the Developers of the Scrum Team. 

Sprint Retrospective is for the Scrum Team (Product Owner, Developers, Scrum Master)

The Scrum Team inspects how the last Sprint went with regards to individuals, interactions, processes, tools, and their Definition of Done. Inspected elements often vary with the domain of work. Assumptions that led them astray are identified and their origins explored. The Scrum Team discusses what went well during the Sprint, what problems it encountered, and how those problems were (or were not) solved.

The Scrum Team identifies the most helpful changes to improve its effectiveness. The most impactful improvements are addressed as soon as possible. They may even be added to the Sprint Backlog for the next Sprint.

The Sprint is a container for all work done, including all of the events.  So since stakeholders are part of the Sprint Review and could be part of Sprint Planning, stakeholders are participants in the Sprint. But their participation is for specific reasons. 

Back to your original question 

Are key stakeholders allowed to participate only in Sprint Review meetings or they can participate in other meetings as well- Spring Planning, Retrospective and Review?

If these are meetings, I would assume that who ever is going to control/facilitate/manage the meeting can decide who should be invited based on the purpose of the meeting.  But if they are meetings, I'd suggest using different names so as not to confuse them with the Scrum events.

Are key stakeholders allowed to participate only in Sprint Review meetings or they can participate in other meetings as well- Spring Planning, Retrospective and Review?

Is the Product Owner unable to represent their interests at these other events? If so, why?