Possible sticking points include whether Palestinian Prime Minister Salam Fayyad head the proposed unity government.
On the agenda are:
Reaching a unified Palestinian strategy;
Forming an interim government;
Reform of the Palestinian Liberation Organisation (PLO);
Setting a date for elections.
Hamas member Ismail Haniyeh was appointed prime minister after the 2006 legislative elections, which his party won, but sacked by Abbas in 2007 when the two parties came into violent conflict.
Hamas has publicly opposed Haniyeh remaining in office, although he has a reputation for efficiency and is said to have tackled some of the corruption that has beset the Palestinian Authority.
The US and the European Union have also declared that they will not work with a coalition government if Hamas does not recognise Israel, renounce the use of arms and agree to abide by previous Palestinian commitments.
Hamas has accused the big powers of trying of exploiting these questions in order to perpetuate the split in Palestinian ranks.
The talks come after Abbas angered Israel and embarrassed the US with a bid for recognition of a Palestinian state at the UN.
If the talks are successful, there will be a follow-up meeting in December. It will involve all Palestinian factions and finalise any agreement reached.
The May agreement called for the immediate formation of an interim government to prepare parliamentary and presidential elections within a year.
It also accepted that Hamas would join the PLO, which has always been dominated by Fatah.