- Since all those technologies use RFID contactless smart card and EMV, are they compatible? Is contact-less Visa card compatible with the Mastercard system?
- Are the systems compatible with NFC? I.e. can the smartphone with NFC controller and antenna emulate e.g. Visa smart card either in SIM card or built-in secure element?
Yes. They are all part of the same association.
Yes. The PayPass, et. al. cards are embedded with an RFID inlay that meets ISO 14443-A and ISO 14443-B standards and operates at 13.56MHz (same as cell phones).
Just because they have the physical layer, however, does not mean that this will be supported for cell phones due to the security risk it represents. Some payment solutions and near-field companies have implemented their own payment systems on top of NFC independent of EMV  .