You would be able to set the visibility of Solution folders to Agents alone or to Everyone, which can be used to create an Internal Knowledge Base.
You could create a New Folder and set it's visibility to Agents. The Solution Articles added to this folder would be visible only to the agents on your account and would be strictly internal.
The default folder which is Visible to Everyone would be the External Knowledge Base, where you could publish articles for both customers and agents.
When you click on a folder, you would find the Edit("Pen" icon) next to the Folder Name. You could click on that to set the Visibility of the Folder accordingly.