These terms are often used confusingly so your question is a good one. Mutagens cause direct or indirect damage to DNA that results in mutations, which are changes in the DNA sequence that are retained in somatic cell divisions and passed onto progeny in germ cells. Mutagenicity refers to a chemical or physical agent’s capacity to cause mutations (genetic alterations). Agents that damage DNA causing lesions that result in cell death or mutations are genotoxins. All mutagens are genotoxic, but not all genotoxins are mutagens as they may not cause retained alterations in DNA sequence.

These terms are often used confusingly so your question is a good one. Mutagens cause direct or indirect damage to DNA that results in mutations, which are changes in the DNA sequence that are retained in somatic cell divisions and passed onto progeny in germ cells. Mutagenicity refers to a chemical or physical agent’s capacity to cause mutations (genetic alterations). Agents that damage DNA causing lesions that result in cell death or mutations are genotoxins. All mutagens are genotoxic, but not all genotoxins are mutagens as they may not cause retained alterations in DNA sequence.

And there are a lot of carcinogens that are not genotoxic at all !!!!!

By the way mutation is not the obliged way to develop cancer...

see for example:

http://pubs.acs.org/doi/abs/10.1021/cr300206t

Actually these two terms bear similar meaning due to identical results finally observed.

Genotoxicity covers a broader spectrum of endpoints than mutagenicity, includes DNA damage assessments. DNA damage are not themselves necessarily transmissible to the next generation of cells, pre-mutagenic.

Mutagenicity refers to the production of transmissible genetic alterations.

Mutagens are agents that changes the genetic makeup. Genotoxicity is the potential of chemical agents to damage 

I agree with Azadi Golbamaki . Genotoxicity covers the part of Mutagenicity , where substance make alterations in genetic material leads to call for  mutagen. 

Pallavi you are right .

Mutagenicity: Ability of the drug to induce genetic damage is assessed in bacteria (Ames test), mammalian cell cultures and in intact rodents. Mutagenic substances capable of increasing the spon­taneous mutation rate by causing changes in the DNA.

Genotoxic substances capable of damaging the DNA and leading to mutations.

May be you can useful from the following article

Genotoxicity is a broader term than mutagenicity;

Genotoxicity: The capability of substances to damage DNA and/or cellular components regulating the conformity of the genome, such as the spindle apparatus, topoisomerases, DNA repair systems and DNA polymerases and includes all adverse effects on genetic information.

Mutagenicity: The capability of substances to cause DNA damage or mutations.