Hello Sriharizzz, and welcome to Seven Forums.
The tutorial below can help show you how to dual boot with Windows 8 and Windows 7.
Dual Boot Installation - Windows 8 and Windows 7 or Vista
However, if this is the same Windows 7 that you used the Windows 8 Upgrade Assistant
to qualify for an upgrade copy of Windows 8, then you are no longer allowed by the EULA
(see below) to use the product key for this Windows 7 when you have the upgrade copy of Windows 8 installed.
If you wish to use both, then you would need to buy a "system builder" copy of Windows 8 instead.
MICROSOFT SOFTWARE LICENSE AGREEMENT
Windows 8 EULA What about upgrading the software?
The software covered by this agreement is an upgrade to your existing operating system software, so the upgrade replaces the original software that you are upgrading. You do not retain any rights to the original software after you have upgraded and you may not continue to use it or transfer it in any way.
This agreement governs your rights to use the upgrade software and replaces the agreement for the software from which you upgraded. After you complete your upgrade, additional software will be required to playback or record certain types of media, including DVDs.
Hope this helps,