XON/XOFF is also known as software flow control (as distinct from hardware flow control, which uses hardware signals).

Telecoms types could also call this "in-band signalling"

"1, What characters are used for Xon or Xoff, especially in Windows hyperterminal program."

ASCII Control Charactes DC1 and DC3 (codes 0x11, 0x13; Ctrl+Q, Ctrl+S) are used (almost?) invariably.
Modems usually have S-Registers to define these control codes, but I wasn't aware that this is configurable in HypoTerminal; I couldn't find it as an option in ProComm.

"2, What if some one sends a data which contain characters similar to Xon or Xoff."

Then you can't use XON/XOFF flow control!

This is why we have protocols like Intel-Hex, Motorola S-Records, MIME, etc for sending binary data over communications links!

"3, How much efficient it is, and how much error imunity it has."

What do you mean by "efficient?"

It isn't recommended for high-speed links, because of the time taken for the software to recognise the XOFF character and respond to it [1].
Clearly, the XON and XOFF characters are just as likely to be corrupted as any other!

"4, Is Xon/Xoff is sent every byte or every chunks."

This is Implementation-dependent, but most likely the receiver will just send XOFF whenever it feels overwhelmed; if the transmitter is not sending data faster than the receiver can handle it, there will be no need to send an XOFF at all!

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[1] Actually, with modern UARTs which can do XON/XOFF in hardware, this is not necessarily such a restriction; but not all UARTs do have this support, and even less have it turned on - so it can't be relied upon, unless you're fully in control of both ends of the link!