I was trying to know more about Apache Camel, Mule ESB, JBoss ESB]...and in general speaking about ESB. I can find several ESB characteristic in Apache Camel. So, can I use Apache Camel like ESB or is Apache ServiceMix is the best fit?
Actually, Apache Camel has to be seen as a part of an ESB as it is strong in the area of routing and mediation (transforming message formats, talking to different protocols).
In order to have a meaningful ESB setup, you also want to have a message broker, e.g. Apache ActiveMQ and a framework which is supporting you in service orchestration (versioning, deployment, ...), e.g. Apache Karaf.
All these technologies (plus some additional projects) are actually bundled in the Apache Servicemix distribution, so if you are looking for an Open Source solution, this might be a good start for you. As far as I know, at least Camel, ActiveMQ and Karaf are core components of enterprise products like JBoss Fuse.
Also see this thread for a more comprehensive discussion.