I want to know, is there any website which offers the comparison of two ICs simultaneously. I don't want to read the whole data sheets. I just want to find the list of differences between two IC's in the same page. For eg: Atmega 8 and Atmega 16.
Sometimes (depending on what you want to compare) you can do this with parametric search functions, either on manufacturer websites or other vendor websites (digikey etc).
Other than that, you have to actually do the hard work and read the datasheets. If you're comparing between two parts from the same vendor like in your example, the datasheets will be formatted in a similar enough fashion that you can probably find the matching information in the same place in each document. If you make use of the bookmarks that are often included in the PDF files (especially for something like a microcontroller), it's really not that bad.