Before continuing - check this benefits the user experience. When I move my mouse over a page and brush over a hyperlink I don't always want a link preview to appear on top. However, assuming this is in the best interests of your users...
Implementation wise, this can be done, as @Michael suggests, by using an <iframe>
, however the document within the iframe will be displayed at the user's set zoom level, but showing a 250x250 window of a document designed for at least 1024x768 isn't going to help the user. Thus you need to display a zoomed-out, birds' eye representation of the web-page to the user.
There are ways to get the current viewport zoom level ( How to detect page zoom level in all modern browsers? ) but I don't know how setting it works (in all liklihood it's probably impossible in most cases). Furthermore I don't think you can set zoom on a per-iframe basis (assuming you can set it all).
The best way forward then is to display a scaled-down bitmap page rendering to the user - like Google does for popular pages in its search results. However this means that for every page you link to you need to get a rendered image of the target page.
I remember a few years ago there were companies that provided page thumbnail services (it was part of those annoying doubly-underlined ad text in webpages that was popular around 2005-2008), but they're a rarity now.
I guess you'll have to then set up your own service and host a layout engine (Gecko, WebKit, or Trident) in a way it can generate page thumbnails for you.
All things considered, I don't think it's worth it.