You're going to have a hard time of it. There's no official support for Solaris in Xen. That said, OpenSolaris users have reported that it works just fine (and that it doesn't work... hence the no official support). Right now, Xen only supports the following: (taken directly from the Xen site @ Citrix.com)
Microsoft Windows 64-bit:
* Windows Server 2003 Standard, Enterprise, Datacenter Edition SP2
Microsoft Windows 32-bit:
* Windows Server 2003 Web, Standard, Enterprise, Datacenter SP0/ SP1/SP2/R2
* Windows Small Business Server 2003 SP0/SP1/SP2/R2
* Windows XP SP2
* Windows 2000 SP4
* Windows Vista
Linux 64-bit:
* Red Hat Enterprise Linux 5.0, 5.1
* CentOS 5.0, 5.1
* Oracle Enterprise Linux 5.0, 5.1
Linux 32-bit:
* Red Hat Enterprise Linux 3.5, 3.6, 3.7, 4.1, 4.2, 4.3, 4.4, 4.5, 5.0, 5.1
* CentOS 4.1, 4.2, 4.3, 4.4, 4.5, 5.0, 5.1
* Oracle Enterprise Linux 5.0, 5.1
* Novell SUSE Linux Enterprise Server 9SP2, 9SP3, 10SP1
* Debian sarge (3.1), etch (4.0)
But... if you really want to do it, it can *theoretically* be done. Mind you, that one word, theoritically, means "Good Luck, it's not likely going to happen in ten languages. Without knowing for certain what applications are in use, I can't get too specific, but since you're talking about two entirely different processor architectures, you probably won't be able to just install-and-run on Solaris guests. Most likely you will need to acquire versions of the applications compiled for x86 servers, build your Xen boxes, create the Solaris x86 guests, install the x86 applications, and test like there's no tomorrow.
Again, if you want to delve into the risky waters:
http://wiki.tardis.ed.ac.uk/index.php/OpenSolaris_Xen_domU
http://www.kernel.sg/roller/eugene/entry/opensolaris_xen_paravirtualized_guest_for
But... the applications probably won't come over without converting to the x86 versions (if any).
Good luck, and let us know how it goes!