I have a kivy app that uses a kv file, main.py, and a py class that handles the database. Everything works fine in windows.  When I run linux (ubuntu) I get a black window with the correct title, but there are no widgets in the window.
What do I need to do different to run a kivy app, that was put together and runs windows, in Ubuntu? I am using the most current available version of kivy on both systems.
It sounds like your kv file isn't being loaded. Does it have the correct name, and is in the right directory? You can check the output in the terminal to see whether the file is loaded.
Edit: One possibility is case sensitivity - windows is not case sensitive, linux generally is. Make sure the kv filename is all lowercase.
Kivy app works on Windows 7 but not on ubuntu