There are fundamental incompatibilities between the ways AWT and Swing draw themselves. Swing classes are pure Java classes that look the same on all operating systems. AWT Java classes are not pure Java classes. They use underlying C/C++ native code (OS dependent) that can cause different appearances in different operating systems, so mixing Swing and AWT defeats the purpose of Swing.
There are no big architectural differences; the class hierarchy is almost the same. The difference lies in the way the two are drawn on the screen. AWT components are heavyweight components with their own viewports, which send output to the screen. Swings are lightweight components and do not write themselves to the screen, but instead redirect the screen to the components it builds on. Heavyweight components have their own Z-ordering. This is the reason why Swing and AWT cannot be combined in a single container. If they are, the AWT components will be drawn on TOP of Swing components.
For example: When AWT components are used with JtabbedPane, they do not disappear when the tabs are switched.