relation between com port, jtag and uart
The only thing relating these three phrases is that they all have something to do with serial communication.

A "COM port" is a serial port specifically in Windows or DOS.  They are the only operating systems which refer to serial ports as COM.

JTAG is a serial programming / data access protocol designed for interfacing with microcontrollers and similar devices.

A UART is a chip or sub-component of a microcontroller which provides the hardware to generate an asynchronous serial stream such as RS-232 or RS-485.

The "COM Port" and "UART" are probably the most closely linked in that a UART will be used in the computer to provide the hardware that Windows sees as a "COM Port".

The JTAG is completely unrelated.