can i install more than one linux on a same machine.........
i.e fedora and Kubuntu on asame machine.....
will it affect a performance??????

I use vmware server to run multiple Linux installations on the same machine. That is easier than dual-boot, and is a marvellous way to set up servers on the same machine. They can be up and running simultaneously.

Whether that works for you depend on how you are trying to access them and what you are trying to accomplish. Say, if you are running a GUI desktop, even one vmware is already slow enough that hurts performance. But for me, as the purpose is to set up servers, say Web server. These virtual hosts can have conventional IPs assigned just like the ordinary hosts.

I have a test J2EE server running in a virtual server this way. Performance is even better than running it on my localhost.

The more pedestrian scenario is dual boot, where you'd have the hard drive partitioned into multiple images, and pick whichever one you want at boot time in the Grub menu. This only affects disk consumption; once an image loads, it gets exclusive access to the hardware.