Q:

What is the difference between SPI and QPI (in embedded)?

In my project, I am using Winbond W25Q64 64-MB Serial flash, When i read datasheet of this serial flash there is two type:

SPI(W25Q64 FLASH ID :- 0xEF4017) and QPI(W25Q64FV FLASH ID :- 0xEF6017).

I know SPI very well, but first time i heard about QPI. So i have some Questions-
1.What is the difference's between SPI and QPI? 
2.Can i use QPI communication type Flash(using SPI)?
3.What is the main feature's of QPI and what is QPI(a Communication Protocol or something else)?

A:

Description of the QPI protocol is part of the datasheet (I have added the link into your question).
Judging from the description there, it does use four data lines for I/O (in contrast to SPI where one line is designated for input and another for output), thus saving clock cycles (compared to standard SPI) as one byte can be transfered in 2 cycles only. Available commands seems to be different too compared to SPI mode.
This chip supports some "extended" SPI modes (called Dual and Quad SPI) where more data lines are used for sending data to master. These are triggered by standard SPI command, but data transfer differs. See the details in the datasheet.
It is not compatible with "standard" SPI, and I am not sure if there is any common MCU supporting this mode. It looks similar to the SD card / SDIO, but I have no idea how much is it (dis)similar.
Yes, you can. The standard SPI (one input and one output data line) seems to be the default interface. The extended SPI modes are used in response to particular SPI commands only and the QPI mode has to be explicitly enabled by respective SPI command (see figure 3 in page 12).
In addition:

Quad SPI and QPI instructions require the non-volatile Quad 
  Enable bit(QE) in Status Register-2 to be set. When QE=1,
  the /WP pin becomes IO2 and /HOLD pin becomes IO3.

...

QE bit is required to be set to a 1 before issuing an “Enable QPI
  (38h)” to switch the device from Standard/Dual/Quad SPI to QPI,
  otherwise the command will be ignored. When the device is in QPI mode,
  QE bit will remain to be 1. A “Write Status Register” command in QPI
  mode cannot change QE bit from a “1” to a “0”.

Advantage is faster data transfer at the same clock frequency. Details of the protocol are well described in the datasheet including timing plots etc.

