will deep linking increase my website pr. Consider that i have 5 back links to my webpage "xyz.html". what is the effect. will this increase my site's pr or only the pr of that page
Much will depend on your linking structure within your site. In most cases a small amount of PR is passed by the incoming link to the targeted page. From there, you can "distribute" PR to other pages within your website by links from that targeted page to those pages. There are many, many variables in how the whole PR distribution system works. But in essence you control your website's PR distribution by how you set up the linking structure among the pages within your website. For example, some webmasters try to focus PR on their home page. They will have incoming links (directories, etc.) all targeted to their home page. Then they will create only the minimum of links necessary from that home page to other key pages within their site. That is how some try to build PR up primarily on their home page and keep it there. Others may want to distribute PR more evenly around their website, so they use a linking structure with more links more evenly distributed among the various pages. It is a "webmaster choice" issue which way you go.
Deep linking increases the pagerank of your target page (xyz.html) and, to a smaller extent, every page it links to. If your target page links to your homepage, your home page will get a small PR boost from the link.