One may have tensions, feeling of being ill-treated, fear of future etc. and all that will result in making him unhappy. That is internal violence.
When one is angry, frustrated, depressed, or bored, one can not be happy. That in a way is harming his own self and is called internal violence.
There can be many examples hatred, mistrust, prejudice, resentment, frustration, negative memories, aimlessness, insecurity indecision, self-enclosure, lack of communication, fears (of losing things, of sickness, of old age, of death, etc.), cynicism pessimism, negativity.