I heard that a child pornography possession is a violent crime ( federal case). Is it true? Thank you very much for your answer.

Child pornography can be both a State and a Federal crime, and can be prosecuted in both court systems under the theory of dual sovreignity (two governments, State and Federal). In most circumstances, the prosecution is sited in one or the other jurisdiction. Good luck.

DISCLAIMER I do not practice law in your State. This answer is provided solely for informational purposes only. This answer does not constitute legal advice, create an attorney-client relationship, or constitute attorney advertising.

It is considered worse than a simple violent crime in most jurisdictions.

According to the FBI, it could be treated as a "violent crime."

http://newark.fbi.gov/dojpressrel/2009/nk080509a.htm

Possession of child pornography without more is not a violent crime nor is it "worse" than a violent crime. The press release cited by Mr. Levy does not say otherwise. The case referred to involved someone who filmed adolescents in sexual intercourse. The agent investigating the crime said it was a violent crime, a statement which is given more to hyperbole than fact. Don't misunderstand. It can be a serious crime and land the accused with years in prison that exceed his life span. Violent it is not.