Hi,
I have a website www.xyz.com which is basically informational site.
PR of this website is 4.

i have another website with subdomain store.xyz.com, which is actually my online store.
The home page of my subdomin is only PR 2 webpage.

When i use google webmasters tool to check the google crawler frequency, i found that the googlebot visits store.xyz.com every 3-4 days, but only crawls 50-60 pages and leaves.

also it keeps visiting the same pages every time. there are some pages that haven't been crawled for last 3 months and there are some pages that have been crawled every 3-4 days.

note: All pages are very well reachable by human as well as by crawler.

can anybody help me knowing googlebot's behaviour.

thnx.

1) Make sure your pages can be reached by the crawler.

2) Increase the PR of your subpages.

they are very well reachable by the crawler.

but can u explain why it keeps visiting the same pages(they have no PR at all) and not new pages?

They do have PR, you just can't see it yet because the public toolbar PR is rarely updated. Make sure you effectively channel PR throughout those pages so that their PR is high enough to be interesting to Google.