A clustered filesystem is one that can handle multiple computers largely independently accessing the same storage. So it has to deal with, for example, the underlying storage can change unexpectedly (because one of the other computers wrote to it).
Compare that to RAID1 (for example, because it's simple to understand): all that does is take the filesystem's request to "write data X to sector Y on device A" and instead write X to sector Y on devices B and C. Or the filesystem's request to "read sector Y on device A" to a read of sector Y on either B or C (which contain the exact same data). It's completely transparent the filesystem—RAID presents a block device that behaves exactly as block devices are expected to.
Basically, a block device is expected to (this isn't a formal definition):
- allow read/write to sectors in arbitrary order
- if you write data D to sector X, then any amount of time later read sector X, it should return D.
RAID devices do that (in fact, they do it better than actual disks). A shared disk, as you'd use for a clustered filesystem, does not: in particular, it violates the second bullet point, as another machine may have written data D₂ to that sector in the meantime.
LVM functions similar to RAID in this respect, as it's just remapping sector numbers, but in a way that preserves the block device behavior. It may, for example, send sectors 1-100 to device A and sectors 101-200 to device B, but that still preserves those behaviors (provided A and B behave, of course).