Is it permissible for women to visit graves?

The visit of women to men, and the visit of men to them, is Islamically permissible by consensus if they are mahram (unmarriageable kin).

If they are non-mahram, the visit is permissible provided there is no khalwa (private seclusion), no fear of fitna (temptation/strife), and no tabarruj (display of adornment), and that there is a need for it, such as maintaining kinship ties or visiting the sick. The Prophet, peace be upon him, visited Duba'ah bint Az-Zubair and Umm As-Sa'ib, and Abu Bakr and Umar visited Umm Ayman, may Allah be pleased with them.

As for women visiting men, it is also permissible under the aforementioned conditions. Umm Ad-Dardaa' visited a man from the mosque community. The hadith: "No man shall enter upon a woman whose husband is absent after this day of mine unless there is with him one or two men," is understood to permit the seclusion of two or three men with a non-mahram woman, with the interpretation that they are a group from whom the occurrence of immorality is unlikely. As for women visiting graves, that has other rulings.