HI.....Dear
I would also like to add comments from my side according to my understanding and knowledge.
In actual, there is a lot much difference between HR and administration but unfortunately smaller organizations or organizations with unclear HR concepts / functions mix it up with each other and thus such posts do arise like HR and Admin manager and thus instead of hiring and seperating these two post, they want to take the work of both these areas from a single person.
It was truly said that HR could perform the tasks of Administration which are of routine nature (e.g. transport management, vehicle problems handling in pharmaceutical organizations, construction or civil matters...etc) but the tasks performed by HR are definately specialized in nature (Like training and development according to the needs, performance appraisal, career management & succession planning, motivating and retaining employees....etc).
As far as the personnel managment is concerned, it is considered to be the old form of HR and older concept in which human side of employees was less viewed or focused.
Hopefully, I am able to clear the difference between all these.
Regards
Khalid