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:
ICE: Su-Fr 22.00-22.00 (UTC);
FXCM/OANDA: Su-Fr 17.00-17.00 (UTC -4 New York);
Forex.com: Su-Sa 17.00-17.00 (UTC -4 New York).
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.