HIV is spread through contact with body fluids. This could be semen, vaginal fluid, blood and breast milk. It is NOT spread through tears, sweat, faces and urine.
So, it can be spread by vaginal sex, anal sex and oral sex. It does not always mean that if you had sex with an infected person, you will definitely get HIV/AIDS but the chances are very high. So one time, can be enough. HIV is spread through blood, by sharing needles if taking drugs, or using unsterilized needles for tattoos and body piercings and blood transfusions.
Pregnant women who are HIV positive can transmit the virus to their babies in the womb and during birth. Also at the time of breast feeding.
When you kiss a woman or she gives you oral sex and she has the virus, this can be transmitted through tiny cuts on the gums or on the penis. These cuts may be so tiny that you do not see it or feel it.
Using a condom or a dental dam are the best possible safe methods. But even here the condom should not break or slip off. If you are using lubrication with the condom, it should be water based and not oil based as oil based lubricants cause the lates to tear.