How Circumcision could reduce the chance for a HIV infection?
This is an interesting question, which has not been answered yet. It is also questionable, if this protective effect is present at all. There are around 40 studies on the topic available which have subsequently undergone meta-analysis. In a meta-analysis all data from recent studies which meet certain quality criteria are analysed together. This gives a much bigger dataset and stronger statistical data. It also helps eliminating the effects of too small test groups.

One of these meta-analysis identified circumcision to have a positive effect on the prevention of HIV infections (reference 1). The authors recommended considering it as a preventive measure.

Two other meta-analysis were more cautious on the topic and either found only a weak link (reference 2) or that only "randomized control studies" (reference 3) would provide clear data. The problem with all these studies is, that they are observational studies, where unknown other effects can play a bigger role than the supposed main effect. It is ethically very difficult to make controlled trials on the transmission of deadly diseases in humans. This is also the reason why all trials on this have been stopped.

There is a hypothesis on how circumcision may help preventing the infection with HIV (when you not get infected in the operation itself due to improper hygienic standards). The skin in the penis contains a large number of immune cells (dendritic cells, CD4/CD8-T-cells) which are the target cells of the HIV virus. It is thought that the virus can more easily penetrate the relatively thin skin of the foreskin. It is also possible, that the surface size is important here, as there are reports which show that men with a larger foreskin also have a larger risk of getting infected. See reference 4.

Another hypothesis has to do with the with the bacteria which live under the foreskin. The number of different strains present is much lower in circumcised men, which led to the conclusion that the dendritic cells, which present these to the immune system have an easier job in the circumcised men and are more likely to prevent the infection with HIV.
See reference 5 and 6 for more information.

References:


Male circumcision and risk of HIV infection in sub-Saharan Africa: a
systematic review and meta-analysis
Male circumcision for prevention of heterosexual acquisition of HIV
in men
HIV and male circumcision--a systematic review with assessment of
the quality of studies.
Male circumcision for HIV prevention: current research and
programmatic issues.
Why Circumcision Lowers Risk of HIV
Male Circumcision Significantly Reduces Prevalence and Load of
Genital Anaerobic Bacteria