If you send a character from the 8051 to your PC running hyperterminal, Hyperterminal won't echo the character you've sent to it - so you won't get a receive character or receive interrupt on the 8051. If you want the 8051 to echo the characters it receives, as to whether you disable receive interrupts is up to you. There can be a receive character coming in whilst you are sending a character - full duplex. The UART has separate pieces of hardware for the receive and transmit tasks. There is also a holding buffer in the UART, when a complete character is received, it is placed into the holding buffer. This gets read when you read SBUF. In the meantime, another character may be being received, but you must read the holding buffer before the next character completes, otherwise you'll loose the first character. There is a similar mechanism for the transmit side.

As for Keil - I have no experience here.