We have 4 sources of Forex data — provided by FXCM, OANDA, Forex.com and ICE:
The difference between FXCM / OANDA / Forex.com / ICE symbols are as follows: ICE symbols are composite, meaning they consist of data received from several dozens of contributors, while FXCM, OANDA and Forex.com are separate Forex brokers. The trading hours also differ:
As for historical data, ICE and OANDA have longer history available.
ICE symbols don't have volume data for daily resolutions.
Forex.com symbols don't have volume data for all resolutions.