Is it possible to achieve all functionality of Qt application by using Qt Quick Components
This assumption is wrong. You can use QML with both/ either C++ and JS.
As for the question in the topic - QtWidgets and QML are significantly different, they do not cover the same GUI questions as of today. If you mean to ask whether QML can do everything that widgets do, the answer is again a "no". A set of QML components imitating widgets is being worked on, but is not in Qt officially yet.
But i had seen that there are many APIs in QML that can be used in place of C++ . Somthing like Qt Mobility etc. etc.