Yes, it is possible to have different views for iPhone 3.5", iPhone 4" and iPad. Also, you can implement different features in every different device. However, iOS User Interface Guidelines recommends that you provide same functionality across different devices.
Preserve the primary functionality of your app, regardless of the device it runs on. Even though one version might offer a more in-depth or interactive presentation of the task than the other, it’s important to avoid making users feel that they’re choosing between two entirely different apps.
In practice, you can programmatically write a bifurcation to build (programmatically) a view using this macros:
#define isIpad() \
([[UIDevice currentDevice] respondsToSelector:@selector(userInterfaceIdiom)] && \
[[UIDevice currentDevice] userInterfaceIdiom]==UIUserInterfaceIdiomPad)
#define isWidescreen ( fabs( ( double )[ [ UIScreen mainScreen ] bounds ].size.height - ( double )568 )<DBL_EPSILON )
#define isIphone ( [ [ [ UIDevice currentDevice ] model ] isEqualToString: @"iPhone" ] )
#define isIpod ( [ [ [ UIDevice currentDevice ] model ] isEqualToString: @"iPod touch" ] )
#define isIphone5 ( isIphone && isWidescreen )
Also, you can create different xib files for every different devices, using the bifurcation and UIViewController message: - (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
, to load the correct file.
Finally, you can use modifiers in the xib filenames to load different resources between iphone and ipad. For example: You have FirstViewController.xib for your iPad UI; you can create a FirstViewController~iphone.xib that will be loaded automatically by iOS if the device is an iPhone (instead of FirstViewController.xib). You can check this answer for more information about this subject.
EDIT
In the case that you have already "iPhone only xib files" and you want to run those on iPad, you should consider to copy each of those files, and convert the copies to iPad target as this or this answers explains . Then, you can re-name this copies ending with "~ipad.xib", that should solve your problem, however, this views will need a layout revision.