At first, Kaiser is in English emperor. König means king.
@ Winter: Austria was not a kingdom in that times but an arch duchy. The Habsburger were Emperor of Germany, Arch dukes of Austria and Kings of Bohemia (among other titles like King of Jerusalem)
Prussia (East and West Prussia) was part of the HRE in the times of emperor Frederic II. of Hohenstaufen (NOT Frederic the Great of Prussia!!!). Times changed and it was not longer part of the Reich. This lasted until the times of the German Federation, where it was part of from about 1848 to 1850 and then from 1867 in the North German federation. BTW Prussia, East and West, was in that times mostly inhabited by Germans (East more than West).
Adler