I know that starting address after reset of Cortex M3 is 0x00, But I got from the Linker configuration file in IAR related to SAM3U4 is the starting address of NVIC is 0x80000. What is the definition of this.
And what is the Boot Memory mentioned in Atmel SAM3U4 datasheet which is before internal flash0

Presumably it holds the ROM code to support SAM-BA.

The way things work is that the boot device is mapped/shadowed at ZERO, and at it's default address. Once the boot vector is read execution branches to the default/natural address. You could compile code with a zero basis if you choose.